POLG2 and progressive external ophthalmoplegia: To date, the few reported heterozygous POLG2 mutations showed a disease onset in adulthood and were linked to (chronic) progressive external ophthalmoplegia ((c)PEO) and cerebellar ataxia leading to the syndrome of “progressive external ophthalmoplegia with mtDNA deletions, autosomal dominant ataxia type 4” (PEOA4, OMIM #610,131) [4].