The efficiency of infection of primary mixed cells such as PBMCs is quite variable because of several different factors (the numbers of CD4+ and CD8+ T lymphocytes, expression of HIV-1 coreceptors, phytohemagglutinin [PHA] activation, cell growth, cell viability); thus, we set up a standardized system to compare the influences of stable and unstable HLA-C alleles on infectivity. This evidence concerns the gene CD4 and infection.