Studies from mice have demonstrated that Emv2, the single‐copy endogenous ecotropic MLV in the commonly used C57BL/6 strain, mediates at least partial central tolerance,50, 56 but does not prevent the induction of strong and protective antitumor responses against the Emv2 Env and its recombinants that are frequently seen in mouse cancer models.50, 57. Here, ERVW-1 is linked to cancer.