The discovery of mutations in CALR, the gene that encodes calreticulin, in a significant proportion of patients with the myeloproliferative neoplasms (MPN) of essential thrombocythemia (ET) and primary myelofibrosis (PMF) drastically altered the molecular landscape of these diseases (Klampfl et al., 2013[4]; Nangalia et al., 2013[6]). Here, CALR is linked to essential thrombocythemia.