CD4 and AIDS: The high prevalence of CXCR4-use among CRF51_01B-infected subjects suggests that CRF51_01B-infected patients could experience faster immunologic decline compared to subtype B-infected patients; a hypothesis supported by preliminary analyses documenting higher proportion of AIDS at presentation, and greater rate of CD4 T-cell decline among CRF51_01B-infected subjects compared to subjects infected with subtype B [14].