Evolutionary relatedness of viper and primate endogenous retroviruses

A retrovirus previously isolated from a tumored Russell's viper is shown by molecular hybridization to be an endogenous virus of this reptilian species. Radioimmunologic techniques revealed that the viper retrovirus is immunologically and, hence, evolutionarily related to endogenous type D retroviruses of Old World primates. These findings extend the number of vertebrate classes possessing endogenous retroviruses and suggest that type D retroviruses may even be more widely distributed in nature than type C retroviruses.