Although the Janus family of kinases (JAK1, JAK2, JAK3, and TYK2) has been extensively characterized and investigated, the role of Janus kinase activation in the pathogenesis and therapy of human malignancies was not fully appreciated until recently when multiple studies identified a recurrent somatic mutation in the JAK2 tyrosine kinase (JAK2V617F) in the majority of patients with BCR-ABL-negative myeloproliferative neoplasms (MPN), polycythemia vera, essential thrombocytosis, and primary myelofibrosis.
Mutations and translocations in the JAK genes leading to constitutively active JAK proteins are associated with a variety of hematopoietic malignancies, including the myeloproliferative disorders (JAK2), acute lymphoblastic leukemia (JAK2), acute myeloid leukemia (JAK2, JAK1), acute megakaryoblastic leukemia (JAK2, JAK3) and T-cell precursor acute lymphoblastic leukemia (JAK1).