The Scientist reports on new support for the mitochondrial DNA mutational theory of aging: "Pigmented neurons in aged human substantia nigra -- the main site of neurodegeneration in Parkinson disease -- contain very high levels of mitochondrial DNA (mtDNA) deletions ... Neurons with the most mtDNA deletions showed defects in cellular respiration, which the authors say may lead to common symptoms of aging, such as the mild Parkinson-like symptoms often observed in older people. ... I think our result in nigra is the most convincing case so far. ... These aged cells often possessed extremely high levels of deletions -- many showed more than 60% deleted mtDNA, which is considered the phenotypic threshold above which respiratory function of the cell becomes impaired." A good job that researchers are already working on ways to replace age-damaged mitochondrial DNA.