Suppressor of cytokine signaling 3, also known as SOCS-3, Cytokine-inducible SH2 protein 3, CIS-3, STAT-induced STAT inhibitor 3, SOCS3 and CIS3, is a protein which is widely expressed with high expression in heart, placenta, skeletal muscle, peripheral blood leukocytes, fetal and adult lung, and fetal liver and kidney. SOCS3 / CIS3 contains one SH2 domain and one SOCS box domain. SOCS family proteins form part of a classical negative feedback system that regulates cytokine signal transduction. SOCS3 / CIS3 is involved in negative regulation of cytokines that signal through the JAK / STAT pathway. SOCS3 / CIS3 inhibits cytokine signal transduction by binding to tyrosine kinase receptors including gp13, LIF, erythropoietin, insulin, IL12, GCSF and leptin receptors. Binding to JAK2 inhibits its kinase activity. SOCS3 / CIS3 suppresses fetal liver erythropoiesis. It regulates onset and maintenance of allergic responses mediated by T-helper type 2 cells. SOCS3 / CIS3 regulates IL-6 signaling. SOCS3 / CIS3 interacts with multiple activated proteins of the tyrosine kinase signaling pathway including IGF1 receptor, insulin receptor and JAK2. SOCS3 / CIS3 could be used as a possible therapeutic agent for treating rheumatoid arthritis.