MT-ATP6 and maternally-inherited Leigh syndrome: Mitochondrial DNA (mtDNA)-encoded ATP6 subunit mutations cause complex disorders with varying and heterogeneous expressions and severity, which range from adult-onset Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) syndrome to a fatal infantile subacute necrotizing encephalomyelopathy, a maternally inherited form of Leigh Syndrome (MILS).