Rodents play an important role in biology and medicine. Since the mouse and rat share many biological characteristics with humans, they are commonly used as model organisms for understanding disease processes and testing treatments. Moreover, it is relatively easy to experimentally manipulate the genetic composition of mice and rats and, thereby, to model human genetic disorders in these animals.

Large numbers of mice can be raised quickly and relatively cost-effectively. Mice have short life spans and relatively short generation times. This makes the mouse an extremely useful experimental system for studying the genetics and biology of human disease. Rats are slightly more similar to humans than are mice, but rats are larger, have a longer generation time, are less well-characterized genetically, and are not as easily genetically manipulated. So while rats are useful in the study of some aspects of biology, mice are now much more...