MT-ND5 is a mitochondrial gene that encodes a subunit of NADH dehydrogenase, the enzyme that catalyzes the initial oxidation of its substrate NADH to NAD and H in the electron transport chain of mitochondria. A mutation in MT-ND5 is associated with a rare genetic disorder that results in a buildup of lactic acid in the body. A researcher hypothesizes that the mutated NADH dehydrogenase has decreased activity but is not completely nonfunctioning and that, by increasing the pool of NADH in cells