Many dog breeds, including mixed breeds, carry genetic mutations for degenerative myelopathy (DM), a late adult-onset disease that begins with loss of coordination and progressive hind limb paralysis. The disease is particularly prevalent in Boxers and Pembroke Welsh Corgis, and is similar to amyotrophic lateral sclerosis (ALS) in people. Approximately two to three years after first signs of DM appear in dogs, loss of muscle function spreads, resulting in complete paralysis. Although the investigators have found that almost all cases of DM in dogs are associated with mutations in a gene called SOD1, it is not understood how these mutations lead to the progressive paralysis that characterizes DM. Ultimately, the research team hopes to develop a treatment that will prevent the onset and progression of disease. In order to do so, a better understanding of the earliest pathology in the muscles and nerves of affected dogs, and how this pathology spreads over time, is needed. This will enable the identification of targets for therapeutic intervention. In this study, the research team hopes to identify the earliest biochemical and structural changes in the central nervous system, muscles and nerves of dogs, and to characterize these changes as the disease progresses.