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 (PubMed:16396496). Interacts with CLOCK (By similarity). Interacts with RAB5IF (By similarity). Expressed by the principal cells of the epididymis. Detected in flagella of epididymal sperm (at protein level). Acetylated on lysine residues. BLOC1S1 is required for acetylation. Acetylated by CLOCK in a circadian manner. Belongs to the complex I NDUFA9 subunit family.