Retroviral envelope proteins mediate receptor recognition and membrane fusion during early infection. Endogenous envelope proteins may have kept, lost or modified their original function during evolution (By similarity). . SU mediates receptor recognition. . TM anchors the envelope heterodimer to the viral membrane through one transmembrane domain. The other hydrophobic domain, called fusion peptide, mediates fusion of the viral membrane with the target cell membrane (By similarity). . This is the function of ERVK13-1 (endogenous retrovirus group K13 member 1, Ensembl gene identifier ENSG00000260565).