CCR5 and infection: R5/X4 dual-tropic and, to a lesser extent, pure X4-tropic viruses, which use CCR5 and/or CXCR4 co-receptors appear later, about 8 to 10 years after the primo-infection, in about 40 to 50% of HIV-1 patients, due to a shift/extension from the R5 to R5/X4 or X4 tropism4–6.