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 (By similarity). Complex I is composed of at least 49 different subunits. A number of isoforms are produced. According to EST sequences. 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.