POLG and mitochondrial DNA depletion syndrome 4a: The second common POLG mutation, p.W748S, usually found in cis with p.E1143G, has been identified in the homozygous state or compound heterozygous with other POLG mutations in patients with various clinical manifestations including early onset Alpers disease, PEO, ataxia, sensory neuropathy, PEO, and dysphagia (http://tools.niehs.nih.gov/polg).