DNA Polymerase gamma (POLG) is an enzyme that replicates and repairs mitochondrial DNA. Mutations in the gene that codes for the catalytic subunit of the enzyme, the POLG gene, are one of the most common causes of mitochondrial disease. POLG-related disorders can have overlapping phenotypes and affect a number of organ systems, and first onset may occur at any age. The disease group can serve as a paradigm for understanding mitochondrial diseases in general.