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. RNAi-mediated knockdown results in mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) deficiency (PubMed:29590638). Mitochondria in the flight muscle shows a wide range of structural abnormalities including dispersed cristae, onion-like swirling membranes, swollen mitochondria, sparse matrix and irregular cristae densities (PubMed:29590638). Eclosion rate is reduced, locomotion and feeding are severely impaired and lifespan shortened (PubMed:29590638). In the brain, there is a pericerebral fat body deficit (PubMed:29590638). RNAi-mediated knockdown in neurons shortens lifespan and results in mild defects in feeding behavior and geotaxis response (PubMed:29590638). Results in lipid droplet accumulation in the glia (PubMed:25594180). RNAi-mediated knockdown in the glia results in no lipid droplet accumulation (PubMed:25594180). RNAi-mediated knockdown in differentiated muscle impairs feeding and locomotion, and shortens lifespan (PubMed:29590638). Belongs to the complex I NDUFS4 subunit family.