In 2005, a single recurrent gain-of-function mutation in the JAK2 tyrosine kinase (JAK2V617F) was discovered in the majority of patients with myeloproliferative neoplasms (MPNs),1, 2, 3, 4 with this discovery allowing precise diagnosis and catalysing the development of several therapeutic JAK2 inhibitors. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.