Inhibition of HIV-1 infectivity by non-cytotoxic concentrations of the P-TEFb inhibitors DRB, seliciclib and flavopiridol. HeLa37 cells were infected with HIV-1p256 and treated with the indicated amounts of (A) DRB, (B) seliciclib and (C) flavopiridol. After 40 hours the cells were fixed, immunostained for HIV antigens and the number of HIV positive cells counted. The number of infected cells (solid circles) was normalized to the control infection and plotted. Cell viability studies were performed in parallel. Values form cytotoxicity studies (open circles) were normalized to the mock treated cells and plotted.