The mutations of the Cu,Zn superoxide dismutase (Cu,Zn-SOD) gene observed in amyotrophic lateral sclerosis (ALS) patients suggest that free radicals play a role in this fatal disease. Free radicals trigger oxidative damage to proteins, membrane lipids, and DNA, thereby destroying neurons. Mutations of the SOD gene may reduce its superoxide dismutase activity, thereby elevating free radical levels. In addition, the mutant SOD protein may function as a peroxidase to oxidize cellular components, and it may also react with peroxynitrite-a product of the reaction between superoxide and nitric oxide-to ultimately form nitrate proteins. The selective degeneration of motor neurons in ALS may be caused by the high level of Cu,Zn-SOD present in and the large number of glutamatergic synapses projecting to these neurons. Free radical-triggered and age-accumulated oxidation may modify the program controlling motor neuron death, thereby initiating apoptosis of motor neurons in young adults.