HERVs are classified based on their homologies to animal retroviruses. Families belonging to Class I are similar in sequence to mammalian Gammaretroviruses (type C) and Epsilonretroviruses (Type E). Families belonging to Class II show homology to mammalian Betaretroviruses  (Type B) and Deltaretroviruses (Type D). Families belonging to Class III are similar to foamy viruses. For all classes, if homologies appear well conserved in the gag, pol, and env gene, they are grouped into a superfamily. There are more Class I families known to exist. The families themselves are named in a less uniform manner, with a mixture of naming based on an exogenous retrovirus, the priming tRNA (HERV-W, HERV-K), or some neighboring gene (HERV-ADP), clone number (HERV-S71), or some amino acid motif (HERV-FRD). A proposed nomenclature aims to clean up the sometimes paraphyletic standards.