Histopathologic features of bone marrow biopsy and aspirate and response to therapy. (A) Bone marrow biopsy section immunohistochemically stained for tryptase (brown color); (B) aspirate stained with Wright-Giemsa; (C) flow cytometric analysis of the bone marrow aspirate. Mast cells are depicted in red in the upper part of the dot plots. (D) Bone marrow biopsy obtained 2 months after initiation of therapy, stained for tryptase. Compare with panel A. (E) Serum tryptase level during the course of treatment. Imatinib mesylate was started at a dosage of 100 mg once daily and increased to 200, 300, and 400 mg daily on days 5, 8, and 12, respectively. Original magnification, × 40 (A-B); and × 10 (D).