The VRC from viral isolates from patients with the lowest CD4 Tcell counts (<100 cells/μl, median 91.15%; IQR:86.1–98.0%) was significantly higher than from patients with highCD4 T cell counts (>500 cells/μl, median 85.19%; IQR:79.8%–91.8%), p = 0.0004 (Mann-Whitneytest), showing that progression to AIDS is associated with an increase in viralfitness. This evidence concerns the gene CD4 and AIDS.