Definition:

Essential thrombocythemia (ET) is one of the myeloproliferative neoplasms (MPNs), a group of clonal stem cell disorders with similarities at the phenotypic and molecular level. ET is characterized by an isolated thrombocytosis and overlaps clinically with polycythemia vera [DS:H00012] and primary myelofibrosis (PMF) [DS:H01605]. The V617F mutation in the tyrosine pseudokinase region of the JAK2 gene is found in 50 - 60% of ET patients. This mutation produces an increased tyrosine kinase activity of JAK2, resulting in uncontrolled cellular growth in the hematopoietic compartment. Calreticulin (CALR) or myeloproliferative leukemia virus oncogene (MPL) mutations occur in approximately 25%, and 3% of ET patients, respectively.