The major use of stem cells has been to study human genetic disease in the mouse. Over 98 percent of human genes are represented by very similar homologous genes in the mouse. Some molecular biologists have jokingly said that we are more than 98 percent mouse, as well as about 40 percent earthworm. This is because throughout the animal kingdom the basic genes necessary for life are replicated in every organism. Our basic system as mammals was built upon earlier reptilian and amphibian gene structures, with only relatively minor modifications resulting in mammals and, ultimately, primates and human beings. Thus, almost any gene that has been identified as defective in a specific human disease has a counterpart in the mouse and, indeed, in most animals. The discovery of the technique for growing mouse embryonic stem cells led to technology called the “knockout mouse.”

This is the way the knockout mouse works. If a scientist believes he or she has identified a possible candidate gene for a human disease, the disease-causing mutation for this particular gene can be cultured with mouse stem cells. Some of the mouse stem cells will pick up this mutated gene by a process called homologous recombination, and thus will carry this same gene defect. These mouse stem cells can then be injected into the blastocyst cavity of a normal mouse, where they become a part of the early inner cell mass of this mouse embryo, which can then develop into an adult mouse that has that gene defect. If the adult knockout mouse with this specific gene defect has the comparable human disease, then you have absolute proof that you’ve identified the specific genetic cause for the disease. The assay system of the knockout mouse, achievable only through this stem cell technology discovered by Evans and Kaufman in 1981, has saved countless lives already by allowing molecular biologists to define specific genes causing a variety of human illnesses.