This family includes envelope protein from a variety of retroviruses. It includes the GP41 subunit of the envelope protein complex from human and simian immunodeficiency viruses (HIV and SIV) which mediate membrane fusion during viral entry. The family also includes bovine immunodeficiency virus, feline immunodeficiency virus and Equine infectious anaemia (EIAV). The family also includes the Gp36 protein from mouse mammary tumour virus (MMTV) and human endogenous retroviruses (HERVs).

1: Crystal structure of the simian immunodeficiency virus (SIV) gp41 core: conserved helical interactions underlie the broad inhibitory activity of gp41 peptides. Malashkevich VN, Chan DC, Chutkowski CT, Kim PS; Proc Natl Acad Sci USA 1998;95:9134-9139. PMID:9689046