 JAK1 and JAK2 are involved in type II interferon (interferon-gamma) signalling, whereas JAK1 and TYK2 are involved in type I interferon signalling.Mice that do not express TYK2 have defective natural killer cell function.Since members of the type I and type II cytokine receptor families possess no catalytic kinase activity, they rely on the JAK family of tyrosine kinases to phosphorylate and activate downstream proteins involved in their signal transduction pathways.