JAK GO:0004718 IMR:0000213 Janus kinase Janus kinases (Jaks) are non-receptor tyrosine kinases. In mammals, the family has four members, Jak1, Jak2, Jak3 and Tyrosine kinase 2 (Tyk2). In Drosophila there is only one Jak kinase, Hopscotch (Hop). molecule_role non-receptor tyrosine kinase