Fig. 1.

Schematic representation of the phylogeny generated from 3,429 U.K. and worldwide HIV-1 subtype B pol sequences. Filled circles represent sequences from the U.K., and open squares represent non-U.K. sequences. Three branching patterns were distinguished: non-U.K. transmission clusters (a), sporadic U.K. infections (b), and U.K. transmission clusters (c). Transmission clusters are clades of sequences from a particular location that descend from a common ancestor, indicating a successful spread of the virus in that location. U.K. transmission clusters are defined as those clades that include at least 25 sequences, 90% or more of which are of U.K. origin.