Adapter protein of the Toll-like and IL-1 receptor signaling pathway that is involved in the activation of NF-kappa-B via MAP3K1. Promotes proteolytic activation of MAP3K1. Involved in the BMP signaling pathway. Required for normal embryonic development (By similarity). As part of the MCIA complex, involved in the assembly of the mitochondrial complex I. Interacts with MAP3K1, SMAD4 and TRAF6. Interacts with SMAD1 only after BMP4-treatment (By similarity). Part of the mitochondrial complex I assembly/MCIA complex that comprises at least the core subunits TMEM126B, NDUFAF1, ECSIT and ACAD9 and complement subunits such as COA1 and TMEM186. Interacts with NDUFAF1. Interacts with ACAD9 (By similarity). Interacts with TRIM59 (By similarity). Interacts with TMEM70 and TMEM242 (By similarity). Belongs to the ECSIT family.