Schematic representation of the proviral form of a retrovirus and its env gene products. A, Genomic structure of a provirus, with the four canonical retroviral genes gag (encoding the virion core proteins), prt (encoding the protease), pol (encoding the reverse transcriptase, RNAseH, integrase), and env (encoding the viral membrane protein). The position of the primers designed to amplify the coding env gene is indicated. B, Linear representation of a retroviral envelope protein and its functional domains. The env gene encodes a polypeptide which is cleaved into two proteins, the surface protein (SU), which is involved in receptor recognition, and the transmembrane subunit (TM), which anchors the whole envelope complex to the membrane and is directly responsible for cell membrane fusion and virus entry. The TM subunit contains three functional domains, the fusion peptide, the immunosuppressive domain and the transmembrane domain. C, Schematic structure of the envelope gene products. C-C: disulfide bound.