HIV envelope glycoprotein gp120 mediates fusion of the viral envelope with the target cell membrane by binding to CD4 and a chemokine receptor. Both CCR5 and CXCR4 physically associate with CD4 and gp120, and are therefore referred to as 'HIV co-receptors'. HIV strains are functionally classified and named according to their specificity for CXCR4 (X4 strains), CCR5 (R5 strains), or both (R5X4 strains).