From the initial report of CALR mutations in MPN, it has become increasingly evident that CALR-positive ET patients possess a different phenotype from their JAK2 V617F-positive counterparts with a younger age, male predominance, higher platelet count, lower hemoglobin, lower leucocyte count, and, perhaps most clinically relevant, a lower risk of thrombosis [20–25]. This evidence concerns the gene JAK2 and essential thrombocythemia.