Non-pathogenic SIV infection of sooty mangabeys (SM), an African "natural" host species, is typically characterized by high virus replication, absence of chronic immune activation [11], and lower levels of CD4+ TCM infection, with the latter finding having been attributed at least in part to lower expression of the SIV co-receptor CCR5 on SM CD4+ TCM [9]. This evidence concerns the gene CD4 and infection.