CD4 and HIV-1 infection: Owing to its importance in gene expression, the expression and availability of P-TEFb is under stringent control, and this review will focus on the various mechanisms by which P-TEFb is regulated in activated CD4+ T lymphocytes and differentiated macrophages, the two major cell types that support productive HIV-1 infection, and in resting CD4+ T lymphocytes, one of the main HIV-1 latency reservoirs.