According to a new phylogenetic analysis, the subtype of HIV that causes the majority of AIDS cases started diverging around 1931. The results of the analysis, which was conducted on the Los Alamos supercomputer 'Nirvana' using sequences from 159 envelope genes, are reported in the 9 June Science (Korber et al., Science 2000, 288:1789-1796). The computation used a molecular clock model presuming a constant rate of sequence change, but similar results were obtained with models that allowed changes in rates of evolution. The 1931 date (with a 95% confidence interval of 1915 to 1941) is when divergence began, but transmission from chimpanzees may have occurred earlier (followed by a period of slow replication and almost no divergence) or later (if multiple strains were transmitted). Poliovirus vaccines cultured in chimpanzee kidney cells have been proposed as a possible mode of transfer to humans. But even with the late-transmission model, it is unlikely that transmission could have occurred as late as 1957, which is when poliovirus vaccination began in Central Africa.