To investigate the role of caspases in APL-induced apoptosis in EL4 and B16 cells, we performed caspase-blocking studies using caspase 8 inhibitor (Z-IETD-FMK) and Caspase 9 inhibitor (Z-LEHD-FMK), followed by TUNEL assay. This evidence concerns the gene CASP9 and acute promyelocytic leukemia.