In humans, mitochondrial DNA (mtDNA) is a 16,569-bp double-stranded circular 
molecule, encoding 37 genes, and is exclusively transmitted from the mother. 
According to the recent findings from many studies of mitochondrial diseases 
caused by nuclear gene mutations, the accumulation of somatic mtDNA mutations in 
tissues has been expected to contribute toward age-associated mitochondrial 
dysfunction and a life span.