Janus kinase-2 (Jak2) is a non-receptor tyrosine kinase implicated in signalling via members of the type II cytokine receptor family (such as interferon receptors), the GM-CSF receptor family (IL-3R, IL-5R and GM-CSF-R), the gp130 receptor family (e.g., IL-6R), and the single chain receptors (e.g. Epo-R, Tpo-R, GH-R, PRL-R). The Jak/signal transducer and activator of transcription (STAT) pathway is a key cellular pathway involved in immunity, cell division, cell death and tumour formation. The Jak/STAT pathway is activated by leptin and the leptin receptor. It results in the phosphorylation of STAT3 and its translocation to the nucleus, where it activates the transcription of various target genes.

Mutations and gene fusions involving Jak2 have been implicated in polycythemia vera, essential thrombocythemia, myelofibrosis, other myeloproliferative disorders and leukaemia.