CALR and essential thrombocythemia: Around 95% of PV patients, 50% to 60% of ET and 39% to 57% of PMF patients are caused by JAK2V617F mutation.1JAK2V617F mutation can lead to hyperactivation of the Janus kinase2 (JAK2)/signal transducer and activation of transcription (STAT) signaling pathway.2 The second common mutation in ET and PMF is CALR exon9 frameshift mutation which has been detected in 25% MPN patients.3, 4 Almost 80% of CALR mutants are type 1 (52bp deletion) or type 2 (5bp insertion), which can induce a common frameshift encoding a novel C terminus with high positive charge.