Mitochondrial enzyme used for testing the toxicity of drugs on critical human oxidative functions and testing replication of mitochondrial DNA. Human pol γ is isolated from mitochondria as a complex containing two subunits, a catalytic subunit, pol γA, of 139 kDa and an accessory subunit, pol γB, of 53 kDa. The catalytic subunit is a family A DNA polymerase with separate polymerase and 3′-5′ exonuclease domains.