What should Christians think about artificial selection and genetic modification?
Where are the ethical boundaries?
AbstractGod created everything “very good” in the beginning (Genesis 1:31). God also created living things so that they could fill all available environmental niches over time (Genesis 1:22,28). Part of this ‘filling’ would be through natural selection, which would be in operation even before the Fall. With the advent of modern biotechnology, humans are now capable of artificially changing nature in ways only dreamed of before. Things that once would have taken centuries can now be done in a few steps over a few seasons. Some of this has been enabled by modern genetic analysis. Seed companies, for example, have vast databases of genetic information on the crops they grow. When a new need arises, say a more salt tolerant wheat, they can pull from that knowledge as they decide which strains need to be crossed in order to, hopefully, produce a crop that best fits the need. Genetic engineering also comes into play. Scientists and commercial operations no longer have to wait for years and years for a new variant to arise. Once they learn the genetic cause behind a trait in one species, they can now engineer that trait into another species by targeting the same gene.
Some of the things being done today, like designer babies, three-parent babies, tissue organoids, or genetically modified foods (GMOs) would have been unthinkable not long ago. But biotechnology also brings with it a whole slew of emerging moral issues that we must deal with. We are in vast, new, and uncharted territories. It is becoming increasingly difficult to navigate this veritable bioethical minefield.
As Christians, we must exercise great care with God’s created world. We believe that man has been given dominion (Genesis 1:28) over the planet, but we must also recognize that it is God’s Creation we are tending. So, naturally, many Christians are alarmed at the technological leaps that might potentially overstep the bounds of human dominion. So, we need to address them from a biblical viewpoint. Some think that modifying organisms is a corruption of God’s created order and so it should be forbidden. But we live in a post-Fall and post-Flood world that has undergone a lot of change (under God’s providence), including genetic change, since Creation. In a sense, ‘nature modifies itself’. In other words, the environment and other factors in nature cause selection pressures. But, of course, this is always within the providence of God anyway. With this in mind, let’s see if we can find some defining boundaries.
In order to understand what we can or cannot do with artificial modification, first we must understand how changes occur in nature independent of human interaction. God may have pre-programmed certain change to arise with no guidance. This could be done through several mechanisms, including recombination bringing about new gene combinations. Also, mutations are constantly occurring and many of these affect the looks or behaviors of living things. Natural selection is the major player and the frequency of genetic variants is always fluctuating based on who can reproduce the most in a given environment. In the end, we have to accept that species can change.
What is a species?
But the definition of the word ‘species’ is quite fluid. Scientists disagree with one another as to what a species exactly is. One of the authors (MC) came across over a dozen different definitions during his university training. According to one author, there are at least 22 such species definitions in use today.1 First, keep in mind that the term ‘species’ is a modern concept. The Bible does not refer to individual species, but instead uses the term ‘kind’ (see later), although we can agree to some extent what a species is. For example, creatures that look different to each other but can still interbreed, such as lions and tigers, are often called different species. Why? Because one of the ways to define species is by the way they look (their morphology). This was largely done before the era of modern genetics and a long time before people started to intentionally crossbreed what appeared to be separate species. Charles Darwin also influenced this concept, because he thought that species changed very slowly, over millions of years. Interbreeding between species is evidence they came from an original created kind only a few thousand years ago, but there is no reason for us not to divide them in different ‘species’ because it is simply convenient for us to do so. How else would we talk about lions and tigers, for example?
Generally, biologists tend to define the word species as ‘a group of organisms with genetic and morphological similarity that can interbreed’. Paleontologists, however, use a different definition, because, by definition, fossils cannot ‘interbreed’! Sometimes nearly-identical creatures may appear at different places in the fossil record. But most paleontologists think that the fossil layers represent millions of years of earth history. Thus, they argue that they cannot be the same species. This is due to an a priori commitment to evolution and the deep time that these layers are supposed to represent. In short, they believe evolution must have occurred during the millions of years that supposedly separate these creatures, and ergo, they cannot be the same species.
What is natural selection?
Many people hear about new species arising via natural selection. One famous example taught at many secular universities is how darker-coloured peppered moths allegedly arose due to the increase of air pollution after the industrial revolution. This leads them to believe that this must be proof of evolution. In one sense, the greatest misconception is hearing the word ‘new’ as it gives the impression that something truly new is being produced.
Natural selection can be defined as a process where the gene pool of a population changes over time because some organisms are better able to reproduce in a specific environment than others, due to an advantage that appears in their genetic background. In other words, natural selection can also be called ‘differential reproduction’. It has nothing to do with life and death, even if Darwin always couched his discussion of the subject in terms of survival. It only has to do with reproduction and the organisms that are the most ‘fit’ are, by definition, the ones that reproduce more. This reproductive advantage drives change in the gene pool over time.
In any group of animals there are genetic variants. Like people, no two organisms are identical. For example, dogs which have genes for longer fur can survive better in a cold environment, whereas dogs with genes for shorter fur could survive better in a warm environment (figure 1). These are extreme examples. Organisms don’t necessarily die during natural selection. All we need is for some to have more offspring in a given environment than others. Thus, natural selection could have worked in the pre-Fall world, where globally it was possible there were some different environments and, additionally, there was no suffering or death, until Adam sinned. Death only becomes a part of the picture in the post-Fall world. In one sense, the genes are ‘selected’ by the environment. It is important to note that natural selection is not a sentient process, whereby ‘mother nature’ knowingly selects the fittest. Scientists use the term ’natural selection’ as shorthand for differential reproduction as determined by the habitat of a creature.
But the environment can only influence reproduction based on what is already in the gene pool. It is possible that, after a while, one of these genetic variations may become ‘fixed’ in a population. At this point, the other variation (e.g. long fur or short fur) has been lost. The species can no longer respond to a reversal of the environmental situation. It has been pigeonholed into a narrower environmental niche than the original population could withstand. At Creation, God designed a system that allowed His creatures to adapt and survive in a post-Fall, and particularly in a post-Flood, world with new environments, and possibly more extreme ones than those that existed in the pre-Flood world.
What role do mutations play in natural selection?
Evolutionists often say “but natural selection plus mutations produces new species”. Often, they will use the term ‘change’, when referring to new species, and this gets interchanged with the word ‘evolution’. They might refer to this as micro-evolution where we see variation between species. But remember that natural selection is only a culling force which cannot turn amoeba into anthropologists (macro-evolution). Thus, term ‘new species’ can be very confusing. Due to the very fluid definition of species, mutations plus natural selection may give rise to ‘new’ species, such as lions and tigers, mentioned earlier. But keep in mind that natural selection cannot create anything ‘new’, genetically speaking. Of course, mutations happen. They often occur during reproduction and are rarely beneficial to an organism. Atheistic scientist and icon of evolutionism, Carl Sagan acknowledged that:
“…mutations occur at random and are almost uniformly harmful—it is rare that a precision machine is improved by a random change in the instructions for making it.”2
Natural selection sorts the genes within populations, allowing individuals to thrive in the environments in which they are most suited. Similarly, mutations cannot create the radically new genetic functions required for upwards evolution, such as new legs for fish to walk out of oceans or new wings to enable dinosaurs to evolve into birds. Natural selection merely sorts existing genetic information, creating different varieties, which allows for adaptation. And, thus, some of these might be classified as new species.
It might be of interest to the reader that the concept of ‘natural selection’ predates Darwin. Carl Linnaeus (1707–1778), a Christian, described it, even though he did not use the term. So did another Christian, Edward Blyth (1810–1873), an English chemist and zoologist, and a contemporary of Charles Darwin. Blyth wrote about natural selection in a very popular journal of the day, when Darwin was still a young man sailing around the world on the HMS Beagle from 1831 to 1836. As far as anyone can tell, Patrick Matthew (1790–1874) coined the phrase in 1831.
Variation within a kind
The discovery of new species should not be a concern for Christians. To understand why, let’s take a closer look at the account of creation in Genesis 1. When the Bible speaks about organisms, it doesn’t use our modern, manmade classifications of science. We shouldn’t read our own species concept into the Bible. Rather, we should let the Bible speak for itself. In Genesis 1:25 we read the following:
“And God made the beasts of the earth according to their kinds and the livestock according to their kinds, and everything that creeps on the ground according to its kind.”
Here the Bible uses a taxonomic unit3 which is different than what we, with our modern mindset, call a species; namely the kind. The kind is broader than a species, and which can include more than one species. Cats, dogs, bears, and dolphins are all examples of different kinds of animals. But each kind can contain many species. For example, the cat kind includes animals such as the lion, the tiger, the leopard, and even the house cat. In fact, there is a chain of genetic and reproductive connectivity among all the members of the cat kind. Dogs were domesticated from grey wolves several thousand years ago. The 400 breeds of domestic dogs have arisen from one subspecies of dog—Canis lupus familiaris. Yet, dogs, wolves (both grey and red), coyotes, and African jackals can all interbreed. Up to 10% of animal species and 25% of plant species can interbreed with at least one other species.4 Each kind has variety in it. Nevertheless, one can usually intuitively determine what ‘kind’ any species belongs to. Cats breed with other cats but will not breed with dogs or sheep or whales.
But sometimes mutations in certain genes may cause the descendants to be incapable of interbreeding with the parent population. Such things involve mutations in egg-sperm recognition factors, or chromosomal rearrangements. This is despite looking very similar to the original group they descended from (like some species of mosquitos, for example). In other cases, even though two species from the same kind technically can interbreed with one another, it is still not likely or feasible in the natural environment. One such technical limiting factor is size. Lions are unlikely to breed with house cats, nor Great Danes with Chihuahuas, for example. None of this is any help to the macro-evolutionary cause, which requires volumes of new encyclopedic information.
Artificial selection and modification
So, new species can arise and change, within the bounds of their created kinds, via natural selection. But what about artificial selection? With artificial selection, environmental factors are replaced by intelligent humans guiding the process. With humans becoming the influencing factor, things take on a whole new dimension, yet with added responsibility.
We have a dominion mandate
In Genesis 1:28, God commands Adam to rule over the natural world:
“And God blessed them. And God said to them, ‘Be fruitful and multiply and fill the earth and subdue it, and have dominion over the fish of the sea and over the birds of the heavens and over every living thing that moves on the earth.’”
We call this the ‘Dominion Mandate’. In Hebrew, the word for ‘subdue’ is כָּבַשׁ (kavash), which means to bring something into bondage or servitude. But the nature of this subjugation should aim to be benevolent. For example, Micah 7:19 describes how the subjugation of our sins is done out of compassion for us. The word for ‘having dominion’ over something in Hebrew is רְדוּ (ra’dooh), which means to subject something to one’s authority. However, this rule is often benevolent in nature, for example Solomon’s peaceful rule (1 Kings 4:24–25), and the dominion of the Messiah (Psalm 72:8).5
Since God created everything good (Genesis 1:31), man must rule kindly; and cannot destroy the natural world.6 Rather, as a good steward of what God had provided, mankind, starting with Adam, was required to tend and keep the entire world, starting with the Garden of Eden (Genesis 2:15). Therefore, taking care of the earth is man’s responsibility. As stewards, we should strive to be found faithful in all that we do (1 Corinthians 4:2).
How does the Dominion Mandate pertain to artificial selection? How far are we allowed to modify nature? Does it always necessarily lead to ruin, or can it produce something useful?
Jesus healed all those who came to Him. By doing so, he was actively reversing the effects of the Genesis 3 curse that affected God’s original, pristine and uncorrupted Creation.7 Likewise, if we can improve the quality of human life by finding, for example, a cure for cancer, we are following Jesus’ example. Thus, artificially modifying bacteria by inserting the human insulin gene into them should be allowed, since the mass-production of insulin helps diabetes patients. Similarly, the production of genetically modified foods with high nutrient content should be permissible, although these kinds of experiments must be performed with great care so as to ensure the production of healthy products and to prevent the release of ecologically problematic species into the wild.
In Genesis 30:25–31:12 Jacob famously increased the number of his flock’s sheep and goats. As part of the deal he made with his uncle Laban, Jacob was to receive all the motley-coloured animals that he could breed. But Laban tried to trick Jacob by taking away all the motley-coloured animals from his flock, thinking that he could cheat Jacob out of his just wages. But God had revealed a secret to Jacob in a dream, where Jacob saw only the motley-coloured male goats breeding with the females. Thus, by restricting which male goats were allowed to breed, Jacob dramatically increased the number of motley-coloured animals. The inheritance of wool colour in sheep is not straightforward, but it is likely that motley wool is recessive8 compared to the dominant solid white colour. Laban did not know that some of the white sheep carried both the dominant and recessive colour variants. Genetically, these are known as heterozygotes.9 However, the numbers do not work out quite as we expect. Even if only the motley-coloured males are breeding, the females could be either pure white, motley, or maybe even pure black. In this case, the lambs and kids would not all be motley, according to the laws of genetics. However, coat colour patterns in sheep and goats are controlled by a complex network of genes, so this is not a matter of simple Mendelian genetics. Also, there is an aspect of Divine Providence here that cannot be discounted. Either way, artificial selection is right there in the Bible.
Artificial selection by breeding can take a very long time. For example, it took the Native Americans in Central America and Mexico several centuries to breed modern corn from its ancestor, teosinte. Teosinte is a grass species with only a single row of kernels on its stalk. After many years of breeding, they were able to create modern corn, with multiple rows of thick kernels.10
But what if we could use biotechnology to rapidly speed up the breeding process. Instead of just using artificial selection, we could use direct genetic modification. The advantage that genetic modification has over artificial selection is that scientists can cut short the whole process. They do not have to wait for new traits to arise via mutation or crossbreeding. Instead, they can directly change desired genes and have all the changes they want immediately. But note that many of these changes are ones we see happening naturally. The only difference is that we can now speed up the process to make it much faster.
Some of the changes, however, would never happen in the wild. Thus, genetic modification demonstrates that complex genetic information cannot arise over millions of years by random chance. Instead large-scale changes are the direct result of intelligent engineering and design, and occur over a short time period, which points to a supernatural Creator.
For example, the groundcherry (Physalis pruinosa) is a wild plant with sweet-tasting fruit. But the fruits are small and sparse and drop to the ground when ripe. Using so-called CRISPR technology,11 the groundcherry has been genetically modified to produce larger fruit that all ripen together and that can withstand being shipped. Instead of waiting for these changes to happen naturally, geneticists simply changed the genes to produce the desired characteristics all at once.12
Of course, GMO foods are not necessarily without danger. If existing control mechanisms which regulate the production of a given protein are disrupted, then that protein might become toxic. Many diseases probably arose after the Fall due to loss of genetic material. When experimenting with GMO foods, we must take great care to make sure that what researchers produce is both safe to eat and safe for the environment.
We could take this concept a step further. What if we introduce a human gene into another organism? Does the resulting organism become partially human? As mentioned earlier, the human insulin gene has been introduced into bacteria, for the mass production of insulin. Two human genes have also been introduced into cows to produce milk that is 80% similar to human breast milk. The bacteria did not become human, and the cows remained the same as any other cow, except that they produced a humanized milk.
However, adding hundreds or thousands of genes from one species of animal to another could lead to deep moral quandaries. Humans and chimpanzees differ from one another in over a thousand genes. What would happen if we ‘humanized’ the genome of a chimpanzee? We might end up with a hybrid human animal-like creature. Dr David P. Barash, emeritus professor of psychology at Washington University advocates the production of human-chimp hybrids, called humanzees, while at the same time lambasting the Christian concept that humans are created in God’s image. Experiments to create ape-man superwarriors were actually performed unsuccessfully by a Soviet veterinary researcher, Ilya Ivanov, during the 1920s in Africa. Fortunately, he failed. Today, we know there are unbridgeable physical and genetic barriers between different animal kinds, but had those not been in place we would be in an entirely different moral sphere today.
We can genetically modify an organism internally. But what about modifying animals externally? What about things like xenotransplants? Xenotransfusions? Xenoexperimentation? What do these things mean, and what is permissible?
At the least invasive level, we have something known as ‘xenoexperimentation’. The suffix ‘xeno-’ means ‘foreign’. Therefore, xenoexperimentation involves running prior experiments on animals (such as mice, chimps or pigs), to test a drug or a new procedure, because human lives are more valuable than animal lives. After tests are successful on animals, the drug may then be tested on humans.
These experiments make possible what is known as ‘knowledge transfer’ from one organism to another. Usually, experimental organisms like yeast, worms, fruit flies, or mice are studied because their genetics are well-known, and researchers can work with them based on a strong knowledge base. This is very common in scientific research and should not raise concerns, besides the possible inhumane treatment of animal subjects.
Despite the usefulness of such experiments, the results don’t always reliably apply to humans, since the physiology of created kinds are still different from one another. A drug may work on chimpanzees but fail with humans. Even though these kinds of experiments are promising, they still must be conducted with care when advancing to the human phase.
An invasive form of artificial modification involves blood transfusion between two species, called ‘xenotransfusion’. For example, some people have proposed blood transfusions between chimpanzees or pigs and humans, on the grounds that our immune systems are similar, and so therefore the animal’s blood could be able to take over the role of depleted human blood.
The idea may seem simple enough, but there are problems. Some people may have a psychological aversion to having blood or an organ from another species inside their bodies. A more significant problem is the danger of rejection of the foreign blood. There is a high chance that the recipient’s immune system will attack the blood cells and try to destroy them. Even between humans, rejection of blood due to blood type incompatibilities is a possibility.
Still, there are some possible solutions to this problem. For example, pigs produce a complex sugar molecule called Gal (galactose oligosaccharide) on the surface of the cells forming their blood vessels. Humans lack this molecule in their blood vessels, so our immune system tries to destroy it if we receive a tissue transplant from a pig. Researchers have successfully bred pigs which do not produce Gal, and in this way it is less likely that blood transfusion patients will not have an immune reaction.13
An even more invasive level of artificial modification involves transplanting tissues (technically called ‘xenografts’) from one species to another. Several animal species, such as pigs, monkeys and kangaroos are being bred to donate solid organs, or parts thereof, to humans, including the liver, kidney, cornea, or even the heart.
The big problem with these procedures is the problem of xenograft rejection by the recipient, similar to the problems with xenotransfusions. Despite these difficulties, several cases of xenograft transplantation from animals to humans have been successful, so this procedure can improve or even save human lives.14
Another form of artificial modification, which raises bioethical concerns even greater than those of xenografts and xenoexperimentation involves the process of ‘humanizing’ animals, creating chimeras. This involves experimentally inserting specific types of human cells or tissues into experimental animals, such as mice, in order to study how those human cell or tissue types behave in another animal. Since a small proportion of the body cells of such mice come from humans, these mice are described as ‘humanized’. Such experiments could help cure disease.15,16 For example, human liver and thymus cells have been inserted into humanized mice to study diseases involving these tissues.
Some people may think that these kinds of experiments may be useful, and we shouldn’t raise too many ethical concerns about them. But there are still ethical problems associated with them. In many cases, human cells, tissues, or organs used in these experiments have been harvested from aborted embryos. So, the ethical issue is similar to that of three-parent embryos, which also involves the abortion of human embryos for their tissues. In some experiments, HIV-infected neurons are inserted into the brain tissue of mice, to study neuroAIDS.17 In this specific case, the question is, do humanized mice gain human consciousness? Also, are humanized mice part human, part mouse? Artificial modification is not a clear-cut issue.
While there is nothing wrong with things like changing the colour of sheep’s wool through artificial selection, researchers must still proceed with great care regarding artificial selection and genetic modification. They must protect the human and animal subjects involved (Proverbs 12:10). Besides the specific example of Jacob and his sheep, artificial selection/modification are not mentioned explicitly in the Bible, so we must use wisdom gleaned from moral and ethical principles based on Scripture.
For example, in the 1970s plant breeders in Australia engineered a new variety of canola (an oil-producing crop) they thought would grow better in Australian environments. However, it was susceptible to blackleg disease, which is caused by a species of fungus that attacks the roots of the plant. During the breeding process, the new variety of canola had lost genes which made the plant resistant to the fungus. Many of the genetically engineered canola plants rotted due to the lack of resistance to the fungus. Plant breeders had to re-introduce the resistance genes into the plant from the original European canola. The new variety had suffered from the loss of too much genetic information.
But what if, through genetic modification, we could produce a variety of wheat that could grow in low rainfall climates and potentially stave off famine in a third-world country? Surely this would be a good thing. Jesus told us to feed the hungry (Luke 3:11), after all.
Conclusion and boundaries
Making bioethical decisions in today’s technological world is difficult. Many inter-related factors are involved. On one hand, we have secular researchers, working within the evolutionary paradigm, who reject biblical values and the idea of man created in the image of the Creator. The evolutionary worldview has at its foundation, the belief that humans are just another evolved species. Thus, it devalues human life and overturns moral boundaries based upon a Christian worldview, which believes that humans are made in the image of their Creator.
This kind of thinking has led leading atheist Richard Dawkins to say that there should be no moral objection to breeding people for certain traits:
“…if you can breed cattle for milk yield, horses for running speed, and dogs for herding skill, why on Earth should it be impossible to breed humans for mathematical, musical or athletic ability?”
Dawkins advocates for breeding people with a specific, desirable trait. If an embryo does not show that desirable trait, then this tiny human being could be relegated to subhuman status, or worse, and if detected in the womb, the embryo could then be aborted. Note how akin this kind of thinking is to Hitler’s concept of breeding a master race. This is certainly way past the ethical boundary of what is permissible and what is not.
Nowadays research involving tissues from continually aborting embryos, or even producing animal-human chimeras is not a problem for some. We should definitely not follow this line of thinking, no matter how noble the therapeutic cause may appear to be, or however incidental suffering or death may be.
On the other hand, we should remain open to certain biotechnological advances which do not involve hurting or killing humans, and which may help increase the quality of human life. For example, to fulfill the dominion mandate that God gave us in Genesis 2, we may use some forms of artificial selection and genetic modification. These are permissible because natural selection already occurs under God’s providence. God even allowed it in one case. We are not taking on the role of a ‘co-Creator’, but simply modifying organisms within bounds.
As stewards of God’s creation, we are free to apply science to reverse the effects of the curse, thereby giving glory to God. But sadly, in a fallen world, everything good can also be used for bad or vain purposes. For example, using artificial modification for purely cosmetic purposes, like changing your baby’s eye color to make it ‘more attractive’, is objectionable and maybe rejected on a ‘vanity’ principle. Another negative example is if human lives (i.e. embryos) are killed in order to develop a new medical procedure. This is morally reprehensible. But gene therapy in the womb might be a good thing if it saves a baby’s life.
In sum, as new technologies develop, we need to exercise biblical wisdom to discern what is acceptable and what is not. This will become increasingly difficult in a world that rejects its Creator. We should aim at drawing boundaries to distinguish between what benefits humankind, following Jesus’ example of reversing the effects of the Curse. In any case, we should first seek to understand and weigh the consequences and the benefits of artificial modification, because it can be used either for great good or for great evil.
References and notes
- Mayden, R.L., A hierarchy of species concepts: the denouement in the saga of the species problem, Species: The Units of Biodiversity, Edited by M.F. Oaridge, H.A. Dawah, and M.R. Wilson, Chapman & Hall, London and New York, pps. 381–424, 1997. Return to text.
- Sagan, C., The Dragons of Eden, Hodder and Stoughton, London, p. 28, 1977. Return to text.
- A taxonomic unit is a grouping of different species such as species, genus, family, order, class, phylum, and kingdom, each larger than the previous one. Return to text.
- Mallet, J., Hybridization as an invasion of the genome, Trends Ecol Evol. 20(5):229–37, 2005. Return to text.
- Sarfati, J., The Genesis Account, Creation Book Publishers, Powder Springs, Georgia, pps. 252–253, 2015. Return to text.
- Dempster, S.G., Dominion and Dynasty, theology of the Hebrew Bible, D.A. Carson (Ed.), InterVarsity Press, Downers Grove, Illinois, 2003. Return to text.
- Robertson, O.P., The Christ of the Covenants, Presbyterian and Reformed Publishing Company, Phillipsburg, New Jersey, 1980. Return to text.
- A recessive gene is a gene (denoted by a lowercase letter, such as a) that can be masked by a dominant gene (denoted by an uppercase letter, such as A). Each gene is present in two copies. A dominant gene needs only one copy to be expressed, whereas a recessive gene can only be expressed in the absence of the dominant gene. Return to text.
- A heterozygote is an animal which has two different variants of a given gene (denoted as Aa). A homozygote is an animal which has two of the same variants of a given gene (denoted as either AA or aa). Return to text.
- Kistler, L., et al., Multiproxy evidence highlights a complex evolutionary legacy of maize in South America, Science 362(6420):1309–1313, 2018. Return to text.
- CRISPR stands for ‘clustered regularly interspaced short palindromic repeats’. This technology involves inserting foreign DNA into a very specific location within the host genome. Return to text.
- Lemmon, Z.H., et al., Rapid improvement of domestication traits in an orphan crop by genome editing, Nat Plants 4(10):766–770, 2018. Return to text.
- Manji, R.A., Lee, W., and Cooper, D.K.C., Xenograft bioprosthetic heart valves: Past, present and future. Int J Surg. 23(Pt B):280–284, 2015. Return to text.
- Cooper, D.K., A brief history of cross-species organ transplantation, Proc (Bayl Univ Med Cent) 25(1):49–57, 2012. Return to text.
- Brehm, M.A., Jouvet, N., Greiner, D.L., Shultz, L.D., Humanized mice for the study of infectious diseases, Curr Opin Immunol. 25(4):428–35, 2013. Return to text.
- Honeycutt, J.B. and Garcia, J.V., Humanized mice: models for evaluating NeuroHIV and cure strategies, J Neurovirol. 24(2):185–191, 2018. Return to text.
- NeuroAIDS is a disease where HIV infects the human central nervous system. Return to text.