Abstract
Janus kinase 2 (JAK2) is a cytoplasmic tyrosine kinase that transduces signals, especially those triggered by hematopoietic growth factors such as erythropoietin, in normal and neoplastic cells. In March and April 2005, four groups of investigators reported finding an acquired JAK2 mutation (termed JAK2 V617F) in association with polycythemia vera and related myeloproliferative disorders.1 These seminal reports have already been cited many times, and JAK2 is now a target for the development of new treatments for the myeloproliferative disorders. JAK2 V617F is detectable in more than 95% of patients who have polycythemia vera, as defined according to conventional criteria. The . . .