ECSIT is an adapter protein of the toll-like and IL-1 receptor signaling pathway that is involved in the activation of NF-kappa-B via MAP3K1. Activation of NF-kappaB as a consequence of signaling through the Toll and IL-1 receptors is a major element of innate immune responses. ECSIT is specific for the Toll/IL-1 pathways and is a regulator of MEKK-1 processing. It bridges TRAF6 to MEKK-1. Expression of wild-type ECSIT accelerates processing of MEKK-1, whereas a dominant-negative fragment of ECSIT blocks MEKK-1 processing and activation of NF-kappaB. ECSIT is also required for normal embryonic development and efficient assembly of mitochondrial NADH:ubiquinone oxidoreductase.