Histologic response with imatinib mesylate therapy (case 3). Hematoxylin-eosin (H&E)–stained (A,D) and tryptase-immunostained (B-C,E) bone marrow biopsy tissue showing the markedly hypercellular marrow with characteristic aggregates of atypical mast cells before treatment with imatinib mesylate (A-C). Panels D-E show the dramatic reduction in marrow mast cell burden and overall cellularity after imatinib mesylate therapy (at 4 weeks). Only scattered tryptase-positive mast cells are observed following therapy. Original magnification, × 80.