In our study and the study by Knox et al. [4], authentic XMRV produced from the human prostate cancer cell line 22Rv1 was used in the neutralization assays, while the studies of Groom et al. [22] and Zhou et al. [24] utilized viruses made with Moloney Gag-Pol proteins and XMRV Env, and were produced by transfection of human 293T cells. This evidence concerns the gene ERVW-1 and prostate cancer.