Between 20 and 25 percent of Leigh syndrome cases are caused by mutations in mitochondrial DNA. The most common of these mutations is found in 10 to 20 percent of Leigh syndrome and occurs in MT-ATP6, a gene that codes for a protein in the last complex of the oxidative phosphorylation chain, ATP synthase, an enzyme that directly generates ATP. Without ATP synthase, the electron transport chain will not produce any ATP. The most common MT-ATP6 mutation found with Leigh syndrome is a point mutation at nucleotide 8993 that changes a thymine to a guanine. This and other point mutations associated with Leigh syndrome destabilize or malform the protein complex and keep energy production down in affected cells. Several mitochondrial genes involved in creating the first complex of the oxidative phosphorylation chain can be implicated in a case of Leigh syndrome, including genes MT-ND2, MT-ND3, MT-ND5, MT-ND6 and MT-CO1.