Inhibited induction of differentiation by ATRA or Vit D3 in RB-transfected U-937 cells as compared with mock-transfected cells. Cells (0.2 × 10⁶ /mL) of the RB-transfected clones (▪) B3 and (▨) B4, both stably overexpressing pRb, as well as mock-transfected clones (□) M2 and (▧) M6 were incubated with ATRA (A) or Vit D3 (B) at concentrations ranging from 0 to 1,000 nmol/L 4 days followed by an NBT-reducing test. Two hundred cells from each clone were counted and the percentage of cells containing reduced NBT was determined. Results shown are mean values from three separate experiments.