CXCR4 and infection: Cell surface CD4 expression is required for the entry of nearly all primary HIV isolates (Dalgleish et al., 1984), and CCR5 (Alkhatib et al., 1996, Deng et al., 1996, Dragic et al., 1996) or CXCR4 (Feng et al., 1996) are essential coreceptors for infection with R5-tropic or X4-tropic HIV isolates, respectively.