This was further corroborated by the discovery that the majority of transmitted viruses are CCR5 utilizing (R5 tropic) [1, 8, 9], although later during natural history of the infection a tropism switch to CXCR4 co-receptor utilizing (X4 tropic) HIV-1 variants occurs in untreated infection in approximately 50 % of individuals [10]. Here, CCR5 is linked to infection.