Phylogenetic analysis of HIV-1 pol, env, and nef sequences. Phylogenetic relationships between pol (A), env (B), and nef (C) sequences from 1995 and 2000 are shown. Open and closed circles represent twin 1-05 sequences from 1995 and 2000, respectively; open and closed triangles represent twin 1-06 sequences from 1995 and 2000, respectively. HXB2, RF, and JR-CSF sequences were used as outgroups for the phylogenetic tree.