Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that affects about 20,000 people in the United States and a significant number of patients in Europe. About 5,000 new cases of ALS are diagnosed each year in the United States. The average life expectancy of ALS patients after diagnosis is about 3 to 5 years, and only about 10% of patients can survive for more than 10 years. As the strength of the skeletal muscles responsible for breathing weakens, the patient eventually develops respiratory failure. Patients currently have limited treatment options and a high degree of unmet need.