Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig's Disease) is a fatal neurodegenerative disorder characterized by selective death of motor neurons in the brain and spinal cord. This leads to muscle weakness, atrophy, and gradual paralysis. At present, there is no effective therapy for the disease and patients usually die within 2-5 years after the onset of symptoms. In order to accelerate the identification of effective treatments for ALS, a biomarker for measuring disease progression is needed.