The exact cause of amyotrophic lateral sclerosis (ALS) is still unknown. However, researchers are studying several possible theories, including gene mutations, overabundance of the neurotransmitter glutamate, autoimmune response, and the gradual accumulation of abnormal proteins in nerve cells. ALS affects the nerve cells that control voluntary muscle movements such as walking and talking. These nerve cells are called motor neurons. There are two groups of motor neurons: upper motor neurons and lower motor neurons. ALS causes both groups of motor neurons to gradually deteriorate and then die. When motor neurons are damaged, they stop sending messages to the muscles, and as a result, the muscles cant function. For about 10% of people with ALS, a genetic cause can be identified. For the rest, the cause is not known. Environmental factors, such as smoking, have been associated with an increased risk of ALS.