More recently, mutations in CALR exon 9 have been reported to occur in 20% to 25% of ET cases and 25% to 35% of PMF cases,[2,4] but CALR mutations are extremely rare in MPNs with the t(9;22)/BCR-ABL1; to date, only 12 cases have been reported. The gene discussed is CALR; the disease is essential thrombocythemia.