Primary myelofibrosis (PMF) is a myeloproliferative disorder mostly caused by gain-of-functiondriver mutations in Janus kinase-2 (JAK2), calreticulin (CALR), or myeloproliferative leukemia virus (MPL).1Persons with these mutations often have additional variations inASXL1, EZH2, DMNT3A, IDH1, andIDH2. The gene discussed is CALR; the disease is primary myelofibrosis.