Mitochondria produce energy in all human cells. Strong mitochondrial mutations cause problems in energy production, resulting in diseases of the muscle, heart, and nervous system. Mutations in mitochondria are also frequently found in multiple cancer types. It is unknown whether these mutatins promote cancer or are only an effect of a cancerous environment. Chen and Stafford show that the cancer mutation pattern in some mitochondrial proteins is more consistent with selection rather than a strictly random pattern.