Stained biopsy from patient 1. (a) Histologic image (hematoxylin/eosin staining) of the breast biopsy showing a diffuse infiltrate of tumor cells with scant cytoplasm and large nuclei with “open” chromatin pattern and distinct nucleoli, consistent with blasts. The blasts infiltrate around small residual acini (black arrows) in a lobule (white arrow). (b) The tumor cells are myeloperoxidase positive by immunohistochemistry. (c) Dim to moderate positivity by immunohistochemical staining for CD56. Size bars: 80 µm.