ERVW-1 and neoplasm: As with other transplantable mouse cell lines15, the elevated expression of endogenous MLV envelope glycoproteins in KPAR cells was probably due to the presence of MLVs with restored infectivity, derived from the replication-defective ecotropic MLV (eMLV) provirus Emv2. Indeed, we isolated an infectious MLV, which we refer to as KPAR-associated retrovirus (KARV), by passaging KPAR supernatant in Mus dunni cells, which became strongly reactive with the endogenous MLV envelope-specific 83A25 antibody (Extended Data Fig. 2g), as well as with serum from KPAR tumour-bearing mice (Fig. 2b).