DNA polymerase gamma is the only DNA polymerase found in the mitochondria and is responsible for mtDNA replication and repair. Mutations in the gene encoding polymerase gamma, POLG1, cause mtDNA depletion or multiple mtDNA deletions resulting in highly heterogeneous group of mitochondrial diseases. POLG1 mutations may account for up to 25% of all adult onset mitochondrial disease. POLG1 mutations have been described in Progressive External Ophthalmoplegia both autosomal dominant and recessive forms, parkinsonism and premature menopause, adult onset cerebellar ataxia, Sensory Ataxia Neuropathy with Opthalmoparesis (SANDO), Alpers Syndrome (childhood onset progressive spastic quadriparesis, progressive cerebral degeneration and liver failure) and other childhood or adulthood onset ataxia syndromes with and without seizures.