Differences in the histologic pattern of bone marrow mast cell infiltration in SMCD-eos patients with and without FIP1L1-PDGFRA fusion. Hematoxylin-eosin (H&E)–stained (A,C) and tryptase-immunostained (B,D) bone marrow biopsy tissue showing the ill-defined, loose aggregates of atypical mast cells associated with intense eosinophilia in cases with FIP1L1-PDGFRA fusion (A-B). Typical well-defined clusters of mast cells are rare and generally small (10-30 cells), and bone marrow mast cell infiltration may be difficult to recognize unless a tryptase immunostain is done for these patients. In contrast, patients without FIP1L1-PDGFRA fusion generally exhibit large, well-defined perivascular and/or peritrabecular clusters or diffuse infiltration of atypical mast cells associated with intense eosinophilia (C-D). Bone marrow mast cell infiltration is easily recognized on H&E-stained sections for these patients. Images were obtained using a Leitz Orthoplan microscope (Wetzlar, Germany) equipped with 160 ×/0.4 objective lenses (Leitz, Wetzlar, Germany).