These data are strikingly consistent with the observed low levels of SIVsmm DNA in CD4+ TCM during non-progressive infection of sooty mangabeys and in long-term non progressors [9], [25], and suggest that a possible common mechanism underlying the lack of disease progression in HIV-infected VNP, LTNPs, and SIVsmm-infected SMs is the relative resistance of CD4+ TCM from direct virus infection. This evidence concerns the gene CD4 and infection.