TAK1 (TGF-beta activated protein kinase 1) is a widely expressed enzyme originally identified as a mitogen-activated protein kinase kinase kinase (MAP3K7). It has since been shown to mediate various intracellular signaling pathways, such as those induced by TGF-beta and members of the Toll-IL-1R (TIR) superfamily, thus acting as an intermediate in both proliferative and innate and adaptive immune responses. TAK1 is normally present in cells in a complex with TAK1-binding protein 1 (TAB1) and either TAB2 or TAB3. Association with TAB1 triggers TAK1 kinase activity by inducing TAK1 autophosphorylation, while TAB2 and possibly TAB3 may contribute to SAPK2a/p38a-mediated feedback inhibition of TAK1 activity.