Interaction of ATRA with pan-HDACi. (A) NB4 cells were treated with 1 μM ATRA for 48 h and/or VPA, MS-275 (MS) or LBH589 (LBH) with indicated concentrations for 24 h. Cells were pre-treated with ATRA or not. PI-stained cells were analysed by flow cytometry, the SubG1 fractions (mean±s.e.m., n=3) are shown. Statistics were done with the two-tailed unpaired Student's t-test, ***P⩽0.001, n.s. not significant P⩾0.05. (B) Lysates were made from cell populations that had been treated with 1 μM ATRA for 48 h and/or 1.5 mM VPA or 100 nM LBH589 for 24 h. Immunoblot was used to detect the protein expression of active/cleaved caspase-3, active/cleaved caspase-9, cleaved PARP, and α-tubulin as a loading control.