Betaretrovirus: A genus of the family RETROVIRIDAE consisting of viruses with either type B or type D morphology. This includes a few exogenous, vertically transmitted and endogenous viruses of mice (type B) and some primate and sheep viruses (type D). MAMMARY TUMOR VIRUS, MOUSE is the type species.Mason-Pfizer monkey virus: A species of BETARETROVIRUS isolated from mammary carcinoma in rhesus monkeys. It appears to have evolved from a recombination between a murine B oncovirus and a primate C oncovirus related to the baboon endogenous virus. Several serologically distinct strains exist. MPMV induces SIMIAN AIDS.Jaagsiekte sheep retrovirus: A BETARETROVIRUS that causes pulmonary adenomatosis in sheep (PULMONARY ADENOMATOSIS, OVINE).Endogenous Retroviruses: Retroviruses that have integrated into the germline (PROVIRUSES) that have lost infectious capability but retained the capability to transpose.Retroviridae Infections: Virus diseases caused by the RETROVIRIDAE.Mammary Tumor Virus, Mouse: The type species of BETARETROVIRUS commonly latent in mice. It causes mammary adenocarcinoma in a genetically susceptible strain of mice when the appropriate hormonal influences operate.