Chronic myeloid leukemia (CML) therapy unmasks effects of coexistent CALR mutation. (A-B) BM biopsy obtained at the time of CML diagnosis. Representative hematoxylin and eosin–stained sections from the core biopsy are shown at ×20 (A) and ×40 magnification (B). (A) Inset shows CD61 immunohistochemical stain highlighting megakaryocytes at ×20 magnification. (C) Wright Giemsa–stained PB smear obtained at time of CML diagnosis. Original magnification ×50. (D) BCR-ABL1 fusion transcript and CALR-mutant allele frequency are quantitated in relationship to blood parameters as a function of time. Normal range of blood parameters is indicated by gray bars. (E-F) BM biopsy obtained at 6 months after initiation of imatinib therapy. Representative hematoxylin and eosin–stained sections from the core biopsy are shown at ×20 (E) and ×40 magnification (F). (F) Inset shows CD61 immunohistochemical stain highlighting megakaryocytes at ×20 magnification.