The majority of MPN driver mutations in CALR are caused by two mutations in exon 9 of the CALR gene, typically occurring in a heterozygous manner: (i) a 52 bp deletion (type I mutation), present in approximately 50% CALR‐mutant patients, and (ii) a 5 bp insertion (type II mutation), present in approximately 30% CALR‐mutant patients. Here, CALR is linked to myeloproliferative disorder.