In patients with ET, 50%–60% has a mutation in the gene encoding the Janus-kinase 2 (JAK2V617F) (5), 20%–25% has a mutation in the endoplasmic reticulum-associated protein calreticulin (CALR) gene (6, 7), and 5% has a mutation in the myeloproliferative leukemia virus oncogene (MPL) encoding the thrombopoietin receptor (8). The gene discussed is MPL; the disease is essential thrombocythemia.