Infectivity of WT and p51↓RNH ± T477A mutant virus in single cycle replication assays. HIV-1 derived from transfection of 293T cells was added to P4R5 indicator cells (5 × 103), and infectivity was assessed 48 h post infection as indicated in Materials and Methods. Black and white bars indicate p51↓RNH - T477A or p51↓RNH + T477A mutant viruses respectively, derived from transfected 293T cells, normalized to 25 ng viral p24 at time of infection. Infectivity was determined after 48 h of culture by fluorescent measurement of β-galactosidase gene expression, as described in Materials and Methods. Data are means ± S.D. from 16 independent experiments. Asterisks (*) indicate statistical significance (p < 0.001) between -T477A and +T477A mutant viruses, calculated using a one-tailed Student's t-test assuming equal variance.