TAB1 was identified as a regulator of the MAP kinase kinase kinase TAK1/MAP3K7, which is known 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. This protein, together with either TAB2 or TAB3, activates TAK1 kinase in response to upstream signals. It has been shown that the C terminal portion of TAB1 is sufficient for binding and activation of TAK1, while a portion of the N terminus acts as a dominant negative inhibitor of TGF beta, demonstrating how this protein can function as a mediator between TGF beta receptors and TAK1.