Janus kinase (JAK) is a family of intracellular tyrosine kinases, ranging from 120-140 kDa, that are involved in the JAK-STAT pathway. They participate in the signalling cascade of cytokines by associating with one of the cytokine receptors, for example, the interferon receptor.
There are four JAK family members, JAK1, JAK2, JAK3 and TYK2. JAK1 and JAK2 are involved in type II interferon (interferon-gamma) signalling, whereas JAK1 and TYK2 are involved type I interferon signalling.
Mutation in JAK2 (V617F) has been associated with myeloproliferative disorders, most specifically with polycythemia vera.
The kinase was initially named JAK after "just another kinase." Later, it was said to be named after Janus, the Thessalian gatekeeper of heaven worshipped by Romans. He is described as having two faces. The kinase is named after him because it has two phosphate-transferring domains (only one of which has a known biological role).