POLG and Parkinson disease: Mutations in the POLG gene can cause either point mutations (through impaired mtDNA proofreading) or deletions (through impaired polymerase activity) in mtDNA.19 The first pathogenic mutations in POLG were identified in families with autosomal dominant PEO (adPEO); however, the spectrum of disease associated with POLG mutations has been expanded to include autosomal recessive PEO, adult onset ataxia, Alpers' syndrome, parkinsonism and premature ovarian failure.87