The identification of acquired CALR mutations in patients with essential thrombocythaemia (ET) or myelofibrosis (MF) has meant that disease-initiating mutations can now be detected in about 90% of all patients with a myeloproliferative neoplasm (MPN). Here, we show that only those CALR mutations that cause a +1 frameshift, thereby altering the carboxy-terminus of calreticulin, promote cytokine independence in vitro; in-frame deletions were not functional, and are unlikely to be the pathogenetic mutation underlying some MPN cases. Expression of the thrombopoietin receptor, MPL, was also necessary for factor-independence. Although the CALR mutations are considered to occur only in JAK2 V617F-negative cases and in a heterozygous state, progenitor genotyping revealed that this is not always true. Notably, CALR mutation-positive MPNs can be polyclonal: in one case, two distinct CALR mutation-positive subpopulations could be identified; in another, separate populations of JAK2 V617F-positive and CALR-mutated cells were present. Mitotic recombination involving chromosome 19 in a third instance resulted in the emergence of a CALR mutation-homozygous subclone. Collectively, our studies demonstrate that occasional patients with CALR mutation-positive ET or MF carry other MPN-initiating genetic mutations (including JAK2 V617F), acquire " secondary mutations" before or after the CALR mutation, or evolve over time to being CALR mutation-homozygous.