CCR5 and HIV-1 infection: CCR5 dependent (R5) viruses predominate at earlier, asymptomatic stages of HIV-1 infection whereas viruses that have acquired the ability to use CXCR4 instead of- or in addition to CCR5 for cellular entry (referred to as X4 or R5X4 viruses, respectively) emerge at later stages of infection in a significant proportion of individuals (reviewed in [20]).