Core subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) which catalyzes electron transfer from NADH through the respiratory chain, using ubiquinone as an electron acceptor (PubMed:31916679, PubMed:33148885). Essential for the catalytic activity and assembly of complex I (PubMed:31916679, PubMed:33148885). a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Core subunit of respiratory chain NADH dehydrogenase (Complex I) which is composed of 45 different subunits (By similarity). Interacts with NDUFAF3 (By similarity). Interacts with RAB5IF (PubMed:31536960). Found in subcomplexes containing subunits NDUFS2, MT-ND1 and NDUFA13 (By similarity). Conditional knockout (KO) in the forebrain neurons results in reduced complex I activity, altered brain energy metabolism, increased locomotor activity with impaired motor coordination, balance and stereotyped behavior, neuroinflammation in cortex and hippocampus, and neuronal death in hippocampus (PubMed:33148885). Conditional KO in skeletal muscle results in development of a progressive myopathy resulting in premature death, muscle degeneration accompanied by increased mitochondrial proliferation and serum lactic acidosis and a decrease in complex I activity, assembly and expression in muscle (PubMed:31916679). Belongs to the complex I 30 kDa subunit family.