A schematic representation of the primary structure of Janus kinases (Jaks), which are made up of FERM, SH2-like, pseudokinase and kinase domains. An alternative nomenclature for the putative domains is as a series of Janus homology (JH) domains. The FERM domain mediates binding to cytokine receptors. Both the FERM and the pseudokinase domains regulate catalytic activity and appear to interact with the kinase domain. Jaks autophosphorylate at multiple sites (P), including two in the activation loop of the kinase domain, but the precise function of these modifications is just beginning to be understood.