(= Janus kinase) Family of intracellular tyrosine kinases (120-140kD) that associate with cytokine receptors (particularly but not exclusively interferon receptors) and are involved in the signalling cascade. JAK is so-called either from Janus kinase (Janus was the gatekeeper of heaven) or 'just another kinase'. JAK has neither SH2 nor SH3 domains.
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