AIDS is often preceded by the co-receptor usage shift of the virus from CCR5 (macrophage-tropic) expressed predominantly at the surface of memory CD4+ T cells to CXCR4 (lymphocyte-tropic) at the surface of activated naïve CD4+ T cells, leading to severe depletion of these cells and impaired regeneration of the immune system [5]. The gene discussed is CD4; the disease is AIDS.