The JAK – or Janus kinases, are intracellular proteins that transduce signals from cytokine receptors via the JAK-STAT pathway. More specifically the JAK family phosphorylates the receptor and thus activates downstream proteins, including transcription factors called STATs (Signal Transducers and Activators of Transcription). These transcription factors then migrate to the nucleus where they regulate transcription of many genes with diverse function, including cell growth, development and differentiation. The broad effect on different cellular processes have made proteins of the JAK family interesting targets for drug development and many JAK inhibitors are under development for treatment of numerous disease, spanning from rheumatoid arthritis to psoriasis.

Together with other kinases the JAK family obviously plays an important role in maintaining proper cell function. Mice that do not express an active protein for all JAK family members exhibit embryonic lethality.

JAK, is an abbreviation for ”Just Another Kinase ”, that was the original name of these proteins when they were first discovered in a large PCR-based screen of kinases. Subsequently they were ultimately published as ”Janus kinase”.