Somatic frameshift mutations in exon 9 of calreticulin (CALR) have been identified in a large proportion of JAK2- or MPL-negative myeloproliferative neoplasm (MPN) patients, including those with primary myelofibrosis (PMF) and essential thrombocythemia (ET) [1,2]. Here, CALR is linked to myeloproliferative neoplasm.