The chaperone and calcium storing protein calreticulin is coded by CALR, and newly identified mutations in CALR are found in respectively 49-70% and 56-88% of JAK2- and MPL-negative patients with essential thrombocytaemia (ET) and primary myelofibrosis (PMF). A total of 41 mutations have been identified, all located to exon 9 which codes the protein's C-terminal. CALR mutations are present only in myeloid malignancies and confer a more indolent disease than JAK2-mutated ET and PMF. CALR mutations as a diagnostic and prognostic tool are promising and the mutations are potential targets for immune therapy.