The novel human retrovirus XMRV has been associated with prostate cancer and chronic fatigue syndrome. The nucleotide sequence of XMRV isolated from humans indicates that the virus is nearly identical with XMRV produced from a human prostate tumor cell line called 22Rv1. This cell line was derived by passage of human prostate tumor tissue in nude mice. Sequence analyses reveal that the genomes of these mouse strains contain two different proviral DNAs related to XMRV. These viral genomes recombined to produce XMRV that has been isolated from humans.