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. This endogenous envelope protein has lost its original fusogenic properties. At the origin, this retroviral envelope protein was localized in the virion. Expressed at higher level in thyroid. Expressed at lower level in adrenal, bone marrow, brain, breast, kidney, ovary, placenta, prostate, skin, testis and trachea. The CKS-17 immunosuppressive domain is present in many retroviral envelope proteins. As a synthetic peptide, it inhibits immune function in vitro and in vivo (By similarity). The CXXC motif is highly conserved across a broad range of retroviral envelope proteins. It is thought to participate in the formation of a labile disulfide bond possibly with the CX6CC motif present in the transmembrane domain (By similarity). Belongs to the gamma type-C retroviral envelope protein family. HERV class-I T env subfamily. The cleavage site does not match the consensus. CKS-17 sequence does not match the minimal active consensus.