In 2005, the discovery of an acquired mutation in Janus kinase 2 (JAK2), a cytoplasmic tyrosine kinase (TK), termed JAK2V617F, in patients with PV and related MPN, provided a unique opportunity for the re-evaluation of the diagnostic criteria describing MPN [4–8]. The gene discussed is TKT; the disease is myeloproliferative neoplasm.