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. Complex I is composed of 45 different subunits. Contains four C-X9-C motifs that are predicted to form a helix-coil-helix structure, permitting the formation of intramolecular disulfide bonds. Belongs to the complex I NDUFA8 subunit family.