JAK2 is a non-receptor tyrosine kinase. It is a number of the Janus kinase family and has been implicated in signaling by members of the type II cytokine receptor family, the GM-CSF receptor family, the gp130 receptor family, and the single chain receptors. JAK2 orthologs have been identified in all mammals for which complete genome genome data are available. Mutations of JAK2 have been implicated in polycythemia vera, essential thrombocythemia, and myelofibrosis as well as other myeloproliferative disorders.