POLG and mitochondrial DNA depletion syndrome 4a: Mutations in POLG give rise to several clinical syndromes, including: Alpers–Huttenlocher syndrome (AHS), myocerebrohepatopathy spectrum (MCHS), myoclonic epilepsy myopathy sensory ataxia (MEMSA), ataxia neuropathy spectrum (ANS), and progressive external ophthalmoplegia (PEO) (Stumpf et al. 2013).