Two retroviral types from each of two subfamilies, the lentiviruses and the oncoviruses, have been associated with human neoplasia. In the lentivirus subfamily of retroviruses (Chapter 4), human immunodeficiency virus types 1 and 2 (HIV-1; HIV-2) induce human diseases involving immunodeficiency and wasting syndromes. In the oncovirus subfamily, human T-cell lympho- tropic viruses (HTLV) types I and II have been associated causally or by association only, re- spectively (IARC, 1996), with human disease. In Figure 12.23 may be seen diagrammatic representations of the genomic structures of the HIV-1 virus and of HTLV-I. Because of exten- sive investigation, many of the functions of the various genes of HIV-1 are known, as evidenced from the figure. The tax and rex genes in HTLV-I may be considered as analogs of the tat and rev genes in HIV-1. Note that these viral genomes are significantly more complicated than those discussed in Chapter 4, involving a variety of oncogenes that had been transduced into a variety of retroviruses, both in the natural state and artificially. The analogous genes—such as tat, rev, tax, and rex—do not appear to have counterparts in the mammalian genome. Furthermore, HIV functions to kill specific cells of the immune system, causing a release of virus, while HTLV infection in cells stimulates cell growth while allowing the production of infectious virus.