Researchers have identified a gene, called matrix metalloproteinase-9 (MMP-9), that appears to play a major role in motor neuron degeneration in amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease. The findings, made in mice, explain why most but not all motor neurons are affected by the disease and identify a potential therapeutic target for this still-incurable neurodegenerative disease.