PLZF-RARα differentiation block in U937 cells is responsive to retinoic acid. (A) U937T:PLZF-RARα cells were induced by tetracycline withdrawal for 5 days and the cell-surface expression of myeloid/monocytic differentiation markers (CD11b, CD33, CD14) was assessed by flow cytometry. Expression was calculated as the fold change in mean fluorescence intensity between PLZF-RARα–expressing and nonexpressing cells. In the absence of ligand (), PLZF-RARα inhibited the expression of CD11b, CD33, and CD14; however, in the presence of 10nM ATRA (■), PLZF-RARα enhances the expression of these markers. (B) Transcriptional response of PLZF-RARα target genes to low-dose ATRA (10nM). Data are expressed as mean ± SEM of 3 independent experiments.