CXCR4 and infection: In contrast to the V1/V2 region, the V3 loop plays an important functional role in viral entry, being the primary determinant of whether CCR5 (R5), CXCR4 (X4), or both CCR5 and CXCR4 (R5X4) can be used as coreceptors to initiate infection (reviewed in [12]).