CASP9 and myeloid leukemia: Whether XIAP can directly interact with procaspase-9 remain elusive, as earlier literature suggested a possibility35, 36, but recent studies emphasize that XIAP can only interact directly with processed capase-9 in either human U-937 myeloid leukemia cells stably overexpressing XIAP or reconstituted caspase-9-Apaf-1 holoenzyme complexes containing fully processed caspase-9 or unprocessed procaspase-927, 37.