Mice are common experimental animals in biology and psychology primarily because they are mammals, and also because they share a high degree of homology with humans. They are the most commonly used mammalian model organism, more common than rats. The mouse genome has been sequenced, and virtually all mouse genes have human homologs. They can also be manipulated in ways that would be considered unethical to do with humans (note Animal Rights). A knockout mouse is a genetically engineered mouse that has had one or more of its genes made inoperable through a gene knockout.
There are other reasons why mice are used in laboratory research. Mice are small, inexpensive, easily maintained, and can reproduce quickly. Several generations of mice can be observed in a relatively short period of time. Mice are generally very docile if raised from birth and given sufficient human contact. However, certain strains have been known to be quite temperamental. Mice and rats have the same organs in the same places, just different proportions.