Diagnostic pathology of clinical case. (A) Two atypical blast populations are seen on bone marrow aspirate smear. One population (arrowhead) is composed of small cells with round nuclei, slightly condensed chromatin, distinct nucleoli, and scant cytoplasm that shows cytoplasmic reactivity with periodic acid-Schiff (PAS) in a blocklike pattern and lacks reactivity with myeloperoxidase (MPO). The other population (arrow) is composed of large cells with irregular nuclei, dispersed chromatin, variably distinct nucleoli, and small-to-moderate amounts of blue-gray cytoplasm that shows cytoplasmic reactivity with MPO and lacks reactivity with PAS. (B) Flow cytometric analysis of this bone marrow aspirate reveals 2 atypical blast populations (one highlighted in purple, one highlighted in red) with distinct CD45 expression and variable antigen expression profiles. The CD45(dim) purple population exhibits uniform expression of B-lymphoid markers CD19 and CD10, uniform stem-cell marker CD34, and variable myeloid marker CD33. The red population shows brighter CD45 expression, exhibits uniform expression of CD33, small subset CD34, variable CD19, and lacking CD10 expression.