Tyk2 (Tyrosine kinase 2) is a 1187-amino acid protein (~131kDa), belongs to the family of non-receptor Janus tyrosine kinases, which also includes Jak1, Jak2, and Jak3. Kinases of the Jak family regulate a spectrum of cellular functions downstream of activated cytokine receptors in the lympho-hematopoietic system. Tyk2 is activated by a variety of cytokines: IFN-alpha, IFN-beta, IL-6, IL-10, IL-12, and IL-13 and promotes IFN-gamma production by Th1-type CD4 cells. Tyk2 can be viewed as a dual-function Jak, mediating both pro-inflammatory and anti-inflammatory cytokine responses. Tyk2 is also an important regulator of lymphoid tumor surveillance.