JAK

Janus kinase (JAK) is a family of signaling molecules that connect the intracellular structures of type I and type II cytokine receptors. It is a non-receptor tyrosine kinase. The JAK family contains four members including JAK1, JAK2, JAK3, and TyK2, and the sequences among the members are relatively conserved.

When a cytokine binds to a receptor outside the cell membrane, the cytokine receptor is activated and transmits the signal to JAK kinase, which phosphorylates and further phosphorylates the downstream molecule STAT. Phosphorylated STAT enters the cell nucleus to control the transcription of cellular genes, thereby affecting the biological functions of the cell.

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