The discovery that most BCR-ABL1-negative myeloproliferative neoplasms (MPN) carry an activating JAK2 mutation has moved this tyrosine kinase into the center of interest and has led to refined diagnostic criteria for these disease entities, development of new prognostic models, and the introduction of JAK inhibitors into clinical practice. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.