A genus in the family retroviridae consisting of exogenous horizontally-transmitted viruses found in a few groups of mammals. Infections caused by these viruses include human B- or adult T-cell leukemia/lymphoma (LEUKEMIA-lymphoma, t-cell, ACUTE, HTLV-I-ASSOCIATED), and bovine leukemia (enzootic bovine leukosis). The type species is leukemia virus, bovine.