MT-ATP6 and Leigh syndrome: The most common of these is the pathogenic m.8993T>G/C mutation in MT-ATP6, encoding the ATP6 subunit of mitochondrial ATP synthase, which is proven to both disrupt assembly of complex V and reduce catalytic activity of the enzyme.2 Classic mitochondrial phenotypes described with MT-ATP6 mutations include maternally inherited Leigh syndrome and neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP).