XIAP inhibition–mediated caspase activation potentiates p53-induced apoptosis by decreasing p53-induced p21. OCI-AML3 cells were treated with various concentrations of nutlin-3a, 1396-11 (3μM), or both, with or without pan-caspase inhibitor IDN-1965 (20μM). p21 and p53 induction and caspase activation were determined by Western blot (A-B) and cell death by annexin V staining (B).