sentences:  The tropism of the virus is determined by the SU protein domain because it is responsible for the receptor-binding function of the virus.The SU domain therefore determines the specificity of the virus for a single receptor molecule.The retroviral glycoproteins are oligomeric complexes that are composed of SU-TM heterodimers, which are made in the endoplasmic reticulum after the translation of the glycosylated Env precursor.