The MT-ND4 gene provides instructions for making a protein called NADH dehydrogenase 4. This protein is part of a large enzyme complex known as complex I, which is active in mitochondria. Mitochondria are structures within cells that convert the energy from food into a form that cells can use. These cellular structures produce energy through a process called oxidative phosphorylation, which uses oxygen and simple sugars to create adenosine triphosphate, the cell's main energy source. Several mutations in the MT-ND4 gene are known to cause Leber hereditary optic neuropathy. Each of these mutations changes a single protein building block in the NADH dehydrogenase 4 protein. One MT-ND4 mutation is the most common cause of Leber hereditary optic neuropathy; it is responsible for about 70 percent of all cases worldwide.