Motor neurons are among some of the most unusual cells in the body becaue of their immense size and their role as the critical link between the motor centers of the brain and the muscles. In addition to their intrinsic biological interest, it is vital that we gain a better understanding of these cells and their pathology, since motor neuron degenerative diseases are lethal disorders that affect young and old and are relatively common. For example, one form of spinal muscular atrophy (SMA) is the most common genetic killer of children in the developed world. Amyotrophic lateral sclerosis (ALS), another form of motor neuron degeneration, is the third most common neurodegenerative cause of adult death, after Alzheimer's disease and Parkinson's disease, and is significantly more common than multiple sclerosis (Motor Neurone Disease Association 1998). Currently, approximately 1 in 500 people in England and Wales who die have a form of motor neuron disease (Motor Neurone Disease Association 1998). Each year, 5000 Americans are diagnosed with ALS, and of these, 10% are under 40 years old. Mouse models of motor neuron degeneration are essential for understanding the causes and mechanisms of motor neuron pathology. These mice are yielding important information that will ultimately lead to treatments and potentially cures for these diseases.