MPL and essential thrombocythemia: Essential thrombocythemia (ET) is a myeloproliferative neoplasm characterized by an elevated platelet count greater than 450 × 109/UL, in the presence of acquired pathogenic mutation such as JAK2 or MPL genes and when other causes of reactive thrombocytosis or other myeloid malignancies such as polycythemia vera, primary myeloid fibrosis, chronic myeloid leukemia, or myelodysplastic syndrome have been excluded.