Mitochondria are essential subcellular organelles containing an extranuclear genome (mtDNA). Mutations in mtDNA have recently been identified as causing a variety of human hereditary diseases. 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.