Accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), that is believed not to be involved in catalysis. Required for proper complex I assembly (PubMed:28671271). 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. This a component of the hydrophobic protein fraction (PubMed:12611891, PubMed:27626371). Interacts with BLOC1S1 (PubMed:22309213). Interacts with SLC2A4 (By similarity). Interacts with CLOCK (PubMed:28985504). Interacts with RAB5IF (PubMed:31536960). Acetylated on lysine residues. BLOC1S1 is required for acetylation (PubMed:22309213). Acetylated by CLOCK in a circadian manner (PubMed:28985504). The disease is caused by variants affecting the gene represented in this entry. Belongs to the complex I NDUFA9 subunit family.