Xenotropic murine leukemia virus-related virus (XMRV) is a gammaretrovirus found in association with human prostate cancer and chronic fatigue syndrome, although these associations are controversial. XMRV shows at most 94% identity to known mouse retroviruses. Here we used XMRV-specific PCR to search for a more closely related source of XMRV in mice. While we could not find a complete copy, we did find a 3,600-bp region of XMRV in an endogenous retrovirus present in NIH/3T3 cells. These results show that XMRV has clear ancestors in mice and highlight another possible source of contamination in PCR assays for XMRV.