CD4 and infection: While the use of an in vitro model of latency is a potential limitation of our study (67), there are striking parallels between models of direct primary cell infection (68, –, 72) and in vivo observations in ART patients, including the following: (i) cells infected with R5 virus have memory phenotype (17, 73, –, 75), (ii) gag usRNA is the predominant viral RNA species (17, 60), (iii) low levels of tat and rev msRNA near the limit of detection of RT-PCR (16, 17), and (iv) loss of surface CD4 can be observed (Fig. 2 and 6).