Catalytic (repeat 2) domain of the Protein Tyrosine Kinase, Janus kinase 2PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Jak2 is widely expressed in many tissues and is essential for the signaling of hormone-like cytokines such as growth hormone, erythropoietin, thrombopoietin, and prolactin, as well as some IFNs and cytokines that signal through the IL-3 and gp130 receptors. Disruption of Jak2 in mice results in an embryonic lethal phenotype with multiple defects including erythropoietic and cardiac abnormalities. It is the only Jak gene that results in a lethal phenotype when disrupted in mice. A mutation in the pseudokinase domain of Jak2, V617F, is present in many myeloproliferative diseases, including almost all patients with polycythemia vera, and 50% of patients with essential thrombocytosis and myelofibrosis. Jak2 is a member of the Janus kinase (Jak) subfamily of proteins, which are cytoplasmic (or nonreceptor) PTKs containing an N-terminal FERM domain, followed by a Src homology 2 (SH2) domain, a pseudokinase domain, and a C-terminal catalytic tyr kinase domain. Jaks are crucial for cytokine receptor signaling. They are activated by autophosphorylation upon cytokine-induced receptor aggregation, and subsequently trigger downstream signaling events such as the phosphorylation of signal transducers and activators of transcription (STATs). The PTKc family is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.