In mitochondria, electrons are transferred from NADH to O2 through a chain of three large enzyme complexes, namely NADH: ubiquinone oxidoreductase (NADH dehydrogenase or complex I), ubiquinol: ferricytochrome c oxidoreductase (cytochrome reductase or complex III), and ferrocytochrome c: O2 oxidoreductase (cytochrome oxidase or complex IV). The function of these enzyme complexes is to link electron transfer with proton translocation out of the mitochondrion. In doing so, they generate a transmembrdneous proton motive force which subsequently drives ATP synthesis by the H+-ATPase (complex V). A broad spectrum of human diseases has been associated with defects in the proton-translocating respiratory chain enzymes of mitochondria. The diseases predominantly affect organs with highest demand for ATP, such as brain and muscle, and are therefore called mitochondrial encephalomyopathies.