. "Some of the most genetically diverse regions of the viral genome are present in Env. They are called variable regions 1 through 5 (V1 through V5). Coreceptor usage of gp120 is determined mainly by the primary structure of the third variable region (V3) in the outer domain of gp120. Binding to CCR5 involves a region adjacent in addition to V3 (By similarity)." . . . . . . . . . . . . . .