HeLa CD4-CCR5/CXCR4-LTR/β-gal cells were infected for one viral cycle (20 h) with 0.1 ng of HIV-1 BaL (A,C) or VN44 (B,D). Viral replication was correlated directly with the transactivation of the Lac-Z gene by the early translated HIV-1 Tat gene. β-galactosidase activity was visualized by incubating cells in the presence of the substrate X-Gal which stained the cells blue after X-Gal degradation by the action of the LTR-driven expression of β-galactosidase. Experiments were performed in triplicate and repeated three times. A representative experiment is shown as mean ± standard deviation. *, P<0.05; **, P<0.001; ***, P<0.0001.