Trypase and MITF are coexpressed in systemic mastocytosis. Immunohistochemistry for tryptase and MITF was performed on bone marrow specimens from patients with systemic mastocytosis and other hematologic diseases. (A-D) Cytoplasmic tryptase staining in bone marrow mast cells in 2 patients with systemic mastocytosis and the D816V KIT mutation (A-B), aplastic anemia (C), and acute lymphoblastic leukemia (D). (E-F) Nuclear MITF staining in mast cells in systemic mastocytosis. Mast cells in aplastic anemia and acute lymphoblastic leukemia (G-H) do not express significant MITF. Mast cells in systemic mastocytosis are spindle shaped and grow in clusters, whereas mast cells in other conditions do not. Photomicrograph images were acquired at room temperature with a Kontron ProgRes 3012 digital camera and Roche Image analysis software with a Zeiss Axiophot microscope (original magnification ×20). Images cropped with Photoshop Elements 2.0 (Adobe).