The discovery of the c.1849G>T mutation leading to the p.Val617Phe (V617F) substitution in JAK2 [1–4] has been a landmark in molecular diagnosis of the myeloproliferative neoplasms (MPN) polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). The gene discussed is JAK2; the disease is essential thrombocythemia.