Accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), that is believed not to be involved in catalysis. Complex I functions in the transfer of electrons from NADH to the respiratory chain. The immediate electron acceptor for the enzyme is believed to be ubiquinone. Binds 1 FAD per subunit. Complex I is composed of 45 different subunits (By similarity). This a component of the hydrophobic protein fraction (By similarity). Interacts with BLOC1S1 (By similarity). Interacts with SLC2A4 (By similarity). Interacts with CLOCK (By similarity). Interacts with RAB5IF (PubMed:31536960). Acetylated on lysine residues. BLOC1S1 is required for acetylation. Acetylated by CLOCK in a circadian manner. Belongs to the complex I NDUFA9 subunit family.