Selective transmission of HIV-1. A. The quasispecies of the chronically infected donor is usually composed of a major viral population (dark blue virions), as well as numerous other minor variants. One of these minor variants (yellow virion) successfully crosses the mucosal barrier to generate the infection of the recipient. B. The neighbor-joining trees of HIV-1 env gp120 nucleotide sequences issued from two mother-infant pairs show the transmission of a single maternal viral variant . Bootstrap values are expressed as percentages per 1000 replicates. Only bootstrap values >50% are indicated. Horizontal branch lengths are drawn to scale, with the black bar denoting 1% divergence. Each symbol denotes a single env sequence; □, maternal sequence; ●, infant sequence.