Nature 413, 299–302 (2001)

The final paragraph of the Methods was inadvertently omitted.

Analysis of HIV sequences Eighteen sequences from the c2v3 region of the env gene of HIV-1 subtype E were derived from viral samples taken from different infected individuals in Thailand at approximately the same time26. Codon alignment and phylogenetic analysis were performed using CLUSTAL27 and PHYLIP28. Twelve of the sequences formed a star phylogeny, suggesting independent evolution, and the founding sequence was determined. Non-synonymous distances were calculated with SNAP29. The good agreement between this data and the prediction of equation (2) suggests that non-synonymous evolution in this gene region is mostly adaptive, an observation supported by many other studies of this region.