Medical Research with Animals | 375 Animals differ from humans, and humans differ from other humans, and hence manifest different responses to the same stimuli, due to the following: • differences with respect to genes present • differences with respect to mutations in the same gene • differences with respect to proteins and protein activity • differences with respect to gene regulation • differences in gene expression • differences in protein-protein interactions • differences in genetic networks • differences with respect to organismal organization (humans and rats may be intact systems, but may be differently intact) • differences in environmental exposures • differences with respect to evolutionary histories These are some of the important reasons why members of the same species often respond differently to drugs and toxins, and experience different diseases To many, these reasons also invalidate the use of animals as predictive models for human disease and drug testing Some courts are now recognizing that animal tests are not relevant to humans (see court cases listed in Further Resources) Similarities Despite the above differences, there is no doubt that animals can be useful in science Numbers 3–9 in Table are ex- amples of how animals can be used in a scientifically viable way Number involves the use of animals for spare parts For example, many people have had a damaged aortic valve replaced by the aortic valve from a pig There is no doubt that pig valves function adequately in humans, and hence this is a scientifically viable use of animals Number includes animals used as factories For example, for decades insulin was harvested from cows and pigs at slaughter Hepatitis B and C viruses and other viruses were grown in nonhuman primates and other animals so scientists could have a convenient reservoir of the virus for study This was before the viruses could be grown in culture More recently, mice have been used to produce monoclonal antibodies Number relates to the fact that researchers frequently use tissues obtained from animals to study basic physiological processes This is sometimes referred to as basic science research, as are numbers and Number is the use of animals for dissection, which most people have experienced in school If a teacher wants a student to learn that nerves, arteries, and veins are found close to one another, dissecting animals will reveal this Number is the issue of using animals as a heuristic, or exploratory, device or as a source from which to get new ideas Of course, if a veterinarian or scientist wants to learn about diseases of cats, she can study cats as in number 8, where other nonhuman animals benefit from animal model use And the final area, number 9, is knowledge for the sake of knowledge alone Numbers 3–9 are scientifically viable ways to use animals in science The animal protection community objects to the