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. Mammalian complex I is composed of 45 different subunits. This is a component of the iron-sulfur (IP) fragment of the enzyme. Male and female mice are fertile but produce smaller litters and pups have a lower neonatal survival rate (PubMed:22474353). While mice are normal during the first 4 months of life, they are prone to rapid onset weight loss and sudden death after this period (PubMed:22474353). They display cardiomyopathy associated with a doubling of heart weight, impaired systolic function and a reduction in functional capacity (PubMed:22474353). Males are most severely affected, with a propensity to develop cardiac failure and diminished survival after 4 months of age (PubMed:22474353). Defects are due to membrane respiratory chain NADH dehydrogenase (Complex I) deficiency (PubMed:22474353). In the knockout experiment described above, mice show a complete knockout of Ndufs6 subunit in heart resulting in marked complex I deficiency, but small amounts of wild-type Ndufs6 mRNA are still present in other tissues, probably due to tissue-specific mRNA splicing, resulting in milder complex I defects (PubMed:22474353). Belongs to the complex I NDUFS6 subunit family.