This gene encodes a complex I assembly factor protein. Complex I (NADH-ubiquinone oxidoreductase) catalyzes the transfer of electrons from NADH to ubiquinone (coenzyme Q) in the first step of the mitochondrial respiratory chain, resulting in the translocation of protons across the inner mitochondrial membrane. The encoded protein is required for assembly of complex I, and mutations in this gene are a cause of mitochondrial complex I deficiency. Alternatively spliced transcript variants have been observed for this gene, and a pseudogene of this gene is located on the long arm of chromosome 19. Part of the mitochondrial complex I assembly (MCIA) complex. The complex comprises at least TMEM126B, NDUFAF1, ECSIT, and ACAD9. Interacts with ECSIT. Interacts with ACAD9. At early stages of complex I assembly, it is found in intermediate subcomplexes that contain different subunits including NDUFB6, NDUFA6, NDUFA9, NDUFS3, NDUFS7, ND1, ND2 and ND3