After the JAK2 V617F mutation, insertion and/or deletion (indel) mutations of CALR exon 9 are the second most common driver mutations in the myeloproliferative neoplasms (MPN) of essential thrombocythemia and primary myelofibrosis and their detection is considered a major diagnostic criterion for these malignancies. The gene discussed is CALR; the disease is myeloproliferative disorder.