CALR^del/+mice develop a myeloproliferative disease. (A) Time course of blood parameters of CALR^del/+ and control mice showing significantly increased platelets (Plt) and normal hematocrit and hemoglobin. A mildly elevated white blood cell (WBC) count is shown and is seen only in some cohorts. The CALR^del/+ mice were CALR^fl/+ Mx1Cre⁺, and the littermate control mice CALR^+/+ were CALR^fl/+ Mx1Cre^– or CALR^+/+ Mx1Cre⁺. (B) Histologic analysis showing megakaryocytic hyperplasia with increased clustering and hyperlobated nuclei without fibrosis. (Left panel) Hematoxylin and eosin (H&E) staining performed on BM from mice at 3 to 4 months after poly I:C injection. (Middle panel) BM staining for reticulin (retic) showing no fibrosis. (Right panel) H&E staining of spleen showing normal splenic architecture. (C) CALR^del/+ mice show increased megakaryocyte colonies. Bar graphs showing significantly increased number of colony-forming unit–megakaryocyte (CFU-MK) colonies in BM. CFU-MK assay was performed in media with or without THPO. (D) CALR^del/+ mice show increased megakaryocytes. Flow cytometry was performed to assess the frequency (Freq) of BM megakaryocytes (MKs) (CD41⁺CD42⁺ or CD41⁺CD61⁺). (E) CALR^del/+ mice show reduced erythroblasts in BM. Flow cytometry was performed to assess the frequency of erythroblasts (CD71⁺Ter119⁺). Means ± standard error of the means (SEMs) are shown. *P < .05; **P < .01; ***P < .001. MNC, mononuclear cell; RBC, red blood cell.