This tree contains 8 sequences sampled from 2 countries (A and B). Tips (HIV-1 sequences) were labelled according to its sampling country. A. If there are no epidemiological links between the two populations A and B, viral sequences will consist of two monophyletic groups, therefore representing distinct epidemics. B. In case that an individual sampled within population B acquired the infection in geographic area A, one branch sampled from population B would cluster within the monophyletic clade of the population A. The migration pattern for each country was estimated by counting "state" (county label) changes at each internal node of the tree by the criterion of parsimony. For each country we counted "exporting" (From) and "importing" (To) migration events. Specifically, as shown in Fig. 1b, a state change (A-B) is counted as an exporting migration event for country A and as importing for B. In our study migration events correspond to mobility of HIV-1 strains or infections and, therefore, inferred exporting or importing migration events are proportional to country-wise mobility of HIV-1 subtype B strains.