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. Mammalian complex I is composed of 45 different subunits. This is a component of the iron-sulfur (IP) fragment of the enzyme. Interacts with BCAP31 and TOMM40; the interaction mediates its translocation to the mitochondria; the interaction with BCAP31 is direct. The interaction with BCAP31 mediates mitochondria localization. Reduces the levels of assembled mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) and the rate of Complex I-driven oxygen consumption (PubMed:18396137, PubMed:25594180). Growth is retarded although activity and behavior are not affected during the first 4 weeks (PubMed:18396137). With aging, results in increasing lethargy, rapid deterioration of motor ability, weight loss followed by death (PubMed:18396137). Mitochondrial ultrastructure is normal, although large subsarcolemmal clusters of mitochondria are present in the soleus (PubMed:25594180). Exhibits glial lipid droplets accumulation in the olfactory bulb and vestibular nucleus prior to the onset of the physical signs of neurodegeneration (PubMed:25594180). Treatment with anti-oxidant treatment ameliorates the phenotype (PubMed:25594180). Conditional knockout in Vglut2-expressing glutamatergic neurons leads to decreased neuronal firing, brainstem inflammation, motor and respiratory deficits, and early death (PubMed:31403401). Conditional knockout in GABAergic neurons causes basal ganglia inflammation without motor or respiratory involvement, but accompanied by hypothermia and severe epileptic seizures preceding death (PubMed:31403401). Conditional knockout in cholinergic neurons has no effect on survival, body weight, or motor function (PubMed:31403401). Belongs to the complex I NDUFS4 subunit family.