Mutations of the thrombopoetin receptor (MPL) gene are detectable in 3–5% of ET and 5–8% of PMF patients.1–3JAK2 and MPL mutations were selected as the major diagnostic criteria for MPNs in the 2008 World Health Organization (WHO) classification.4 Recently, mutations of the CALR gene were found in 50–80% of JAK2 and MPL mutation-negative ET and PMF patients.5, 6 Because of this high mutation frequency, detection of CALR mutations is already widely included in the diagnostic programme for MPN. The gene discussed is CALR; the disease is essential thrombocythemia.