MPO and acute basophilic leukemia: According to the current WHO classification [24], there are three diagnostic requirements for acute basophilic leukemia: (1) blasts and mature/immature basophils with metachromatic granules (as shown by toluidine blue stain); (2) blast cytochemistry negative for myeloperoxidase (MPO), Sudan Black B, and non-specific esterase (ANAE); and (3) the absence of strong CD117 expression.