Mitochondria are essential subcellular organelles containing an extranuclear genome (mtDNA). Mutations in mtDNA have recently been identified as causing a variety of human hereditary disease. In most of these cases, the tissues of the affected individual contain a mixture of mutant and normal mtDNA, with this ratio determining the severity of symptoms. Stochastic factors alone have generally been believed to determine this ratio. Jenuth et al.(1), however, examining mice that contain a mixture of mtDNA types, show evidence of strong selective forces at work in favoring one mtDNA variant over another in some tissues.