CALR and myeloproliferative neoplasm: “Classic” Philadelphia chromosome‐negative myeloproliferative neoplasms (MPN) are a heterogeneous group of clonal disorders of hematopoiesis presenting during the adulthood and characterized by the proliferation of one or more myeloid lineages, bone marrow (BM) hypercellularity with effective cell maturation, frequent peripheral cytoses, and recurrent molecular alterations in JAK2, CALR, and MPL genes.