Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND), or Lou Gehrig’s disease, is a specific disease which causes the death of neurons controlling voluntary muscles. Some also use the term motor neuron disease for a group of conditions of which ALS is the most common. ALS is characterized by stiff muscles, muscle twitching, and gradually worsening weakness due to muscles decreasing in size. This results in difficulty speaking, swallowing, and eventually breathing.