The mechanism by which immune stimulation increases circulating levels of HIV-1 is not known. In particular, it is uncertain whether the transient increase in plasma HIV-1 RNA is due to enhanced replication of an actively replicating pool of HIV-1, or is due instead to activation of proviral sequences in previously resting CD4+ cells. One approach to discriminate these alternatives is a "molecular pulse-chase" experiment. In this approach, drug resistant mutants would be selected by administration of Lamivudine (3TC).