Effects of As₂O₃ on APL cell growth. (A) Primary BM cells from one relapsed APL patient were incubated with As₂O₃ (0.1 to 2.0 μmol/L) and ATRA (0.1 μmol/L) for 9 days. Fresh APL cells from five other patients had similar results (not shown); (B) ATRA-susceptible NB4 cells and ATRA-resistant–MR2 cells were incubated with As₂O₃ (1 μmol/L) and ATRA (0.1 μmol/L) for 5 days. Viable cell numbers were calculated with typan-blue exclusion. Every point represents the mean of triplicated samples.