In 2011, Paprotka, et al. reported that XMRV likely originated through recombination between 2
endogenous murine retroviruses, PreXMRV - 1 and PreXMRV - 2, during in vivo passaging of the human prostate cancer xenograft CWR - R1, resulting in establishment of the XMRV - infected 22Rv1 cell line [38].