Self and neighboring protection scenarios of the V1V2 loop. The V1V2 loop can theoretically shield the V3 loop by neighboring protection (model 1), in which the V3 loop of a neighboring gp120 subunit is protected, or by self protection (model 2), in which the V3 loop of the own subunit is protected. Circles denote gp120 monomers. Blue rectangles denote the V3 loop. Gray shaded areas depict the V1V2 loop.