JAK is an ancronym for: just another kinase. This is a family of tyrosine kinases involved in membrane bound receptor signaling. Ligand binding, such as cytokines and growth factors, to their respective receptor appears to be associated with JAK phosphoylation of the same receptor. Ultimately, cytokine binding leads to receptor aggregation, subsequent phosphorylation of the JAK protein as well as the cytosolic region of the membrane bound receptor; generating an active receptor complex.

One of the most common complexes is with the STAT protein. The combination of receptor chains, Janus kinases, and STAT proteins may play a critical role in defining biological specificity seen in certain cytokine responses, as the activation of this complex leads to targeted gene regulation in the nucleus.