The envelope glycoprotein complex of replication competent retroviruses is comprised of two polypeptides, an external, glycosylated, hydrophilic polypeptide (SU) and a membrane-spanning protein (TM), that form a knob or knobbed spike on the surface of the virion. Both polypeptides are encoded in the env gene and are synthesized in the form of a polyprotein precursor that is proteolytically cleaved during its transport to the surface of the cell. While these proteins are not required for the assembly of enveloped virus particles, they do play a critical role in the virus replication cycle by recognizing and binding to specific receptors (SU) and by mediating the fusion of viral and cell membranes (TM): virus particles lacking envelope glycoproteins are thus noninfectious.