Core subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I) that is believed to belong to the minimal assembly required for 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). a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Exons b and c and d and e are cis-spliced, while trans-splicing reactions are required to link exons a and b and c and d. Belongs to the complex I subunit 1 family. It is uncertain whether Met-1 or Met-7 is the initiator.