Schematic representation of retroviral TM subunits, indicating the lengths [in amino acids (aa)] of their respective cytoplasmic tails. As illustrated, lentiviral TM CTs tend to be significantly longer than those of other retroviruses, with the exception of feline immunodeficiency virus (FIV). HIV-1, human immunodeficiency virus 1; SIVagm, simian immunodeficiency virus from African green monkey; EIAV, equine infectious anemia virus; RSV, Rous sarcoma virus; MoMLV, Moloney-MLV; M-PMV, Mason-Pfizer monkey virus; HTLV-1, human T-cell lymphotropic virus type 1. Figure reproduced with permission of Elsevier from Ref. 17.