In the majority of patients, viral variants using CCR5 as coreceptor (R5) remain present in the viral quasispecies and coexist with viral variants using CXCR4 (X4) and variants capable of using both CCR5 and CXCR4 (R5X4) during the remaining course of infection [7,11]. The gene discussed is CXCR4; the disease is infection.