The Suppressor of cytokine signaling (SOCS) and cytokine-inducible SH2 proteins are a family of intracellular proteins that regulate responses to cytokines. SOCS1 suppresses dendritic cell and T cell hyperactivation in response to cytokine signaling by inhibiting the JAK tyrosine kinase. JAK tyrosine kinase activity is important for type I and II cytokine receptors to initiate signaling, and its activity is inhibited by direct binding of SOCS1 to the catalytic domain of the kinase. SOCS1 also has E3 ubiquitin protein ligase activity that results in the polyubiquitination of its target proteins and subsequent degradation by the proteosome.