CD4 and infection: Because the CCR5 receptor is expressed predominantly on memory cells, and its level of expression is highly sensitive to the form of T cell activation used during in vitro culture [74], it is extremely likely that had we been able to use a CCR5-tropic HIV clone to establish infection in our model of latency, the results obtained may have reflected quite different ratios for the prevalence of latent infection in CD4 T cells with minimal activation and cell proliferation.