A mouse (plural mice) is a small mammal that belongs to one of numerous species of rodents. The best known mouse species is the common house mouse (Mus musculus). It is found in nearly all countries and, as the laboratory mouse, serves as an important model organism in biology, and is also a popular pet. The American white-footed mouse (Peromyscus leucopus) and the deer mouse (Peromyscus maniculatus) also sometimes live in houses. These species of mice live commensally with humans.