CALR and essential thrombocythemia: In addition, mutations in the thrombopoietin receptor gene (MPL) have been found in 10–25% of patients with PMF and post-ET MF [6], whereas mutations in the calreticulin gene (CALR) have been observed in JAK2- and MPL-negative patients with PMF (75%) and post-ET MF (50–60%) [7].