Summary Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the loss of motor neurons in the spinal cord and brain. While autosomal dominant mutations in several known genes can cause familial ALS, most ALS cases (90-95%) display no family history so are classified as sporadic. Mechanisms driving sporadic ALS are poorly understood, hindering the search for treatments and a cure. Somatic mutations are DNA variants that arise after fertilization and are thus mosaic in the body, and somatic mutations have been shown to be causal in several non-inherited neurological disorders. Our proposal aims to test whether somatic mutations, in ALS risk genes or other genes, in the spinal cord or brain can cause sporadic ALS. To test this hypothesis, we will use an interdisciplinary approach, combining the expertise in the clinical features and genetics of ALS from the laboratory of Robert Brown DPhil., M.D. with expertise in somatic mutations and single-cell analysis in the lab of Michael Lodato, Ph.D. We will analyze somatic mutations in a consortium-generated whole-genome sequencing (WGS) dataset, and analyze somatic mutations in ultra-deep (500x) whole-exome sequencing (WES) data generated in this proposal. These experiments will allow us to study somatic mutations impacting sporadic ALS at all embryonic stages, including early somatic mutations distributed across the body, and late embryonic somatic mutations that might occur in a committed central nervous system progenitor and be restricted to the brain and spinal cord. Our preliminary data suggest that all donors are marked by somatic mutations, some of which generate predicted deleterious changes. If successful, we will open a new frontier in ALS genetics, and define new patient cohorts who are candidates for recently developed, mutation-specific personalized therapeutics.