Generation of ATO-resistant cell lines. (A) The NB4 naïve parental cell line was exposed to 50 nM ATO for 3 months, and the concentration was gradually increased to 1 µM ATO over a period of 1 year until the cell population was sustained and proliferated. Limiting dilutions were used and colony-forming unit assays were performed to generate monoclones of the resistant cell lines. (B) The bar graph represents the percentage of viable cells after 48 hours of 2 µM ATO. (C) Representative dot plots and stacked bars (that summarize the dot plot results) for NB4 and resistant cell lines were treated with 0.5 µM and 1 μM of ATRA for 72 hours as single agents and in combination with 2 μM ATO. The percentage of differentiation was measured by the surface expression of CD11b, and dead cells were measured by 7-aminoactinomycin D (7-AAD). Graphs and statistical parameters were generated from 3 independent experiments. ****P ≤ .0001.