Laboratory and pathologic features of this MPN with concurrent BCR-ABL1 and CALR mutation during the course of therapy with dasatinib. (A) The patient had marked leukocytosis and no significant thrombocytosis during his initial presentation to us. The WBC count continued to decrease as the patient received dasatinib, whereas the platelet count demonstrated an upward trend. The BCR/ABL1 to ABL1 ratio was >100% at presentation (B-C) and dropped to a nadir of 0.07% at last follow-up, whereas the allele frequency of the CALR mutation remained in the range of 51% to 68% (B). CALR mutation screening was performed using PCR followed by standard capillary electrophoresis on a Genetic Analyzer (Applied Biosystems, Foster City, CA) and Sanger sequencing for confirmation. The frequency of the mutant allele (red arrow) was calculated by dividing the mutant peak area by the sum of the mutant and wild type peak areas. (D) The initial bone marrow (BM) biopsy at presentation to our institution showed a hypercellular bone marrow with marked granulocytic hyperplasia and variably distributed megakaryocytes including a mixture of small and large forms, some with hyperchromatic nuclei (yellow arrows) and others without significant nuclear hyperchromasia (blue arrowheads). The morphologic findings were in keeping with the diagnosis of chronic myeloid leukemia, although the presence of occasional large, hyperchromatic megakaryocytes was unusual. (E-F) A repeat bone marrow biopsy after 7 months of therapy with dasatinib showed a moderately hypercellular BM with prominent megakaryocytic clustering and mild osteosclerosis (hematoxylin and eosin [H&E] stain, original magnification ×100). (G) Notably, there was a predominance of large megakaryocytes with hyperlobulated nuclear contours and prominent nuclear hyperchromasia (H&E stain, original magnification ×100). (H) A reticulin stain showed moderate reticulin fibrosis characterized by coarse bundles of reticulin with many interconnections (H&E stain, original magnification ×100; inset, reticulin; original magnification ×100).