ATRA and FK228 have combined effects on MDR1 mRNA induction in NB4 cells. Induction of MDR1 mRNA expression in NB4 cells treated with the indicated agents at different concentrations. MDR1 mRNA expression in each sample was normalized to B2M mRNA expression. ATRA-treated cells were used as the calibrator (MDR1 mRNA level in ATRA-treated NB4 cells = 1) because of the lack of detectable MDR1 mRNA expression in control NB4 cells. Graphs show the mean ± SD of results from 3 independent experiments. A statistically significant difference derived from the fold-increase numbers was determined by ANOVA and Fisher post hoc tests (^**P < .05).