The DNA damage theory of aging proposes that aging is a consequence of unrepaired accumulation of naturally occurring DNA damage. Damage in this context is a DNA alteration that has an abnormal structure. Although both mitochondrial and nuclear DNA damage can contribute to aging, nuclear DNA is the main subject of this analysis. Nuclear DNA damage can contribute to aging either indirectly (by increasing apoptosis or cellular senescence) or directly (by increasing cell dysfunction). Several review articles have shown that deficient DNA repair, allowing greater accumulation of DNA damage, causes premature aging; and that increased DNA repair facilitates greater longevity. Mouse models of nucleotide-excision–repair syndromes reveal a striking correlation between the degree to which specific DNA repair pathways are compromised and the severity of accelerated aging, strongly suggesting a causal relationship. Human population studies show that single-nucleotide polymorphisms in DNA repair genes, causing up-regulation of their expression, correlate with increases in longevity. Lombard et al. compiled a lengthy list of mouse mutational models with pathologic features of premature aging, all caused by different DNA repair defects. Freitas and de Magalhães presented a comprehensive review and appraisal of the DNA damage theory of aging, including a detailed analysis of many forms of evidence linking DNA damage to aging. As an example, they described a study showing that centenarians of 100 to 107 years of age had higher levels of two DNA repair enzymes, PARP1 and Ku70, than general-population old individuals of 69 to 75 years of age. Their analysis supported the hypothesis that improved DNA repair leads to longer life span. Overall, they concluded that while the complexity of responses to DNA damage remains only partly understood, the idea that DNA damage accumulation with age is the primary cause of aging remains an intuitive and powerful one. In humans and other mammals, DNA damage occurs frequently and DNA repair processes have evolved to compensate. In estimates made for mice, DNA lesions occur on average 25 to 115 times per minute in each cell, or about 36,000 to 160,000 per cell per day. Some DNA damage may remain in any cell despite the action of repair processes. The accumulation of unrepaired DNA damage is more prevalent in certain types of cells, particularly in non-replicating or slowly replicating cells, such as cells in the brain, skeletal and cardiac muscle. DNA damage and mutation To understand the DNA damage theory of aging it is