AbstractA novel human lymphotropic virus capable of crippling the immune system by infecting and destroying T4 antigen‐positive cells is now known to be the etiologic agent of the acquired immune deficiency syndrome (AIDS). The AIDS or human immunodeficiency virus (HIV) belongs to a family of RNA viruses called retroviruses. Several strains of HIV have been molecularly cloned, and DNA sequence comparisons have established that the proviral DNA genome is 9.7 kilobase pairs. The genome possesses characteristic retrovirus features including structural genes, flanked by long terminal repeats, in the ordergag, pol, andenvand, in addition, four unique nonstructural genes, several of which appear to be essential in regulating virus replication. Electron microscopy has played an important role in elucidating structural, genetic, and molecular properties of HIV and has aided in its classification as a member of the Lentivirnae retrovirus subfamily. Heteroduplex mapping methodologies pertinent to these findings are described. Although the relationships show considerable divergence, the similarities between HIV and lentiviruses are profound and encompass an indistinguishable morphology, genome sequence homology and topography, genomic diversity, and overlapping biology, including a preference for infecting cells of the immune system, a cytopathic effect in vitro, and the ability to produce a persistent, slowly progressing, degenerative disease in vivo. The newest HIV class (HIV‐2) has recently been molecularly characterized. HIV‐2 also bears all the hallmarks of a lentivirus but is more closely related to simian immunodeficiency viruses than the previously described HIV‐1, despite a similar biology. The HIV‐lentivirus phylogenetic relationship has broad implications for the AIDS disease process and has given new importance to the study of the natural history and pathogenesis of animal lentiviruses in searching for clues to prevent the sprea