Schematic diagram of subtype-specific nested multiplex PCR. The upper part of the figure is the gag gene region diagram of subtypes B, C, CRF01-AE, CRF07-BC, and CRF08-BC (10). The lower part of the diagram shows the primer sets in nested multiplex PCR. Primer pair GagF2/Gage2 are universal primers for HIV-1 M group strains. Primer sets cn-gag-B1/cn-gag-b2, cn-gagC1/cn-gag-c3, cn-gag-E2/cn-gag-e3 are subtype-specific primers for subtypes B, C, and CRF01-AE, respectively. Primer set cn-gag-C1/cn-gag-BC is specific for CRF07- and CRF08-BC.