We also show that SIVcpz associated immunodeficiency can be effectively treated with antiretroviral therapy, which decreased plasma viremia in Cotton to undetectable levels within 7 weeks and increased CD4+ T cell counts to above 500 cells/μl within 2 years after onset of therapy, although low plasma drug concentrations resulted in the emergence of FTC resistance mutations. This evidence concerns the gene CD4 and immunodeficiency disease.