A small number of complex I subunits are the products of mitochondrial genes (subunits 1-7), while the remainder are nuclear encoded and imported from the cytoplasm. NADH dehydrogenase subunit 5 (ND5) is most likely a component of the hydrophobic protein fragment of Complex I. Mutations in the gene encodiing for ND5 are implicated in Mitochondrial neurogastrointestinal encephalomyopathy, an autosomal recessive multisystem disorder that results in dramatically elevated levels of circulating thymidine and deoxyuridine.