CD4 and HIV-1 infection: HIV-1 infection requires two sequential and specific binding steps: first, to the CD4 antigen present in CD4+ T-cells, monocyte/macrophages and other cells; and second, to a member of the chemokine receptor subfamily, within the G protein-coupled, seven-transmembrane domain family of receptors, mainly CCR5 and/or CXCR4.