Neurodegeneration is the progressive loss of structure or function of neurons, including the death of neurons. Many neurodegenerative diseases – including Parkinson's disease, Alzheimer's disease, and Huntington's disease and amyotrophic lateral sclerosis, – occur as a result of neurodegenerative processes. The greatest risk factor for neurodegenerative diseases is aging. Mitochondrial DNA mutations as well as oxidative stress both contribute to aging. Many of these diseases are late onset, meaning there is some factor that changes as a person ages for each disease. One constant factor is that in each disease, neurons gradually lose function as the disease progresses with age. It has been proposed that DNA damage accumulation provides the underlying causative link between aging and neurodegenerative disease