The mitochondrial theory of aging is the idea that age-associated mitochondrial dysfunction is caused by accumulation of somatic mutations in mitochondrial DNA (mtDNA). However, mitochondria are considered to be a dynamic organelle that repeats fusion and fission. Through fusion and fission, there is an extensive and continuous exchange of mtDNA and its products between mitochondria. This mitochondrial complementation prevents individuals from expression of respiratory dysfunction caused by pathogenic mutant mtDNAs. Thus, the presence of mitochondrial complementation does not support the mitochondrial theory of aging. Moreover, the presence of mitochondrial complementation enables gene therapy for mitochondrial diseases using nuclear transplantation of zygotes. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.