Primary lateral sclerosis (PLS) is a rare, neuromuscular disorder that affects the central motor neurons and is characterized by painless but progressive weakness and stiffness of the muscles of the legs. Such weakness may progress to affect the arms and the muscles at the base of the brain (bulbar muscles).

Juvenile primary lateral sclerosis is caused by mutations in a gene called ALS2. Although researchers don’t understand how this gene causes the disease, they know that the ALS2 gene gives instructions for creating a protein called alsin, which is present in motor neuron cells.