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. May be the terminally assembled subunit of Complex I. Complex I is composed of 45 different subunits. This is a component of the flavoprotein-sulfur (FP) fragment of the enzyme. Belongs to the complex I NDUFV3 subunit family.