Model for cytokine signal transduction. Cytokines associate withcytokine receptor subunits and activate janus kinases (Jaks). The Jaks in turnphosphorylate tyrosine-based docking sites on the receptor. signal transducersand activators of transcription (Stats) then bind via their src homology (SH)2 domains. The STATs are then phosphorylated by the JAKs, formhomo-hetero-dimers and then translocate into the nucleus, where they bindtarget sequences like γ activated sequence (GAS) motif. Transcriptionalactivation of genes typically requires the coordinated function of multipleaccessory transcription factors. Additionally, serine phosphorylation of someStats may be important for maximal transcription of target genes.