CXCR4 and infection: Over 80% of HIV-1 transmissions are initiated with R5 viruses [2-6], with X4 or dual-tropic viruses that use both CCR5 and CXCR4 (R5X4) emerging and coexisting with R5 viruses in 40-50% of non-treated subtype B and D infected individuals late in infection, but less often in subtype A and C infected patients [2,7-11].