Oxidative stress is emerging as a key factor in the pathogenesis of AD.[64] Reactive oxygen species (ROS) over-production is thought to play a critical role in the accumulation and deposition of amyloid beta in AD.[65] Brains of AD patients have elevated levels of oxidative DNA damage in both nuclear and mitochondrial DNA, but the mitochondrial DNA has approximately 10-fold higher levels than nuclear DNA.[66] Aged mitochondria may be the critical factor in the origin of neurodegeneration in AD.[65] Even individuals with mild cognitive impairment, the phase between normal aging and early dementia, have increased oxidative damage in their nuclear and mitochondrial brain DNA[67] (see Aging brain).