Mice, like many other vertebrate species, carry multiple copies of retrovirus-related sequences as part of their normal genetic complement. These endogenous retroviral DNA sequences are, in general, structurally and functionally similar to proviruses formed via exogenous virus infection. They behave as relatively stable genetic entities, are transmitted in a normal mendelian manner, and can be expressed by normal cellular mechanisms. Chromosome mapping studies have shown that these virus-related sequences are dispersed throughout the mouse genome, in most cases were acquired after speciation but before inbreeding, and occasionally cause mutations as a result of integrating within the mouse genome.