Bone marrow biopsy from a patient with AML and myelofibrosis arising from chronic eosinophilic leukemia. Bone marrow biopsies are shown before (A-B) and after (C-D) imatinib treatment. (A) Marrow biopsy section (Ai; hematoxylin and eosin; original magnification, × 4) is hypercellular with scattered eosinophils (Aii; original magnification, × 20) and columnar arrays of immature myeloid cells (Aiii; original magnification, × 20). (B) Reticulin stain highlights severe fibrosis (magnified view, Bii; original magnification, × 10). After 3 months of imatinib therapy, (C) marrow biopsy reveals marked hypocellularity without increased immature myeloid cells or eosinophils, and (D) reticulin stain shows markedly diminished fibrosis (original magnification, × 4 for panels C and D). After an additional 3 months, the patient relapsed with bone marrow findings similar to those in panels A and B. Screening of the FIP1L1-PDGFRA fusion at the time of relapse revealed the interval development of an imatinib resistance mutation (T674I) within the PDGFRA gene.