Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease and motor neurone disease (MND), is a specific disease that causes the death of neurons which control 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 in speaking, swallowing, and eventually breathing. The cause is not known in 90% to 95% of cases. About 5–10% of cases are inherited from a person's parents.