Expanded hexanucleotide repeats in the C9ORF72 gene were identified in Oct 2011 as a cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), thus identifying the single most frequent genetic cause of each and connecting them to repeat expansion disease. We are developing stem cell disease models to enable key hypotheses of pathogenesis and new interventions to be tested. We have successfully engineered stem cell models to analyze the effects of C9ORF72 mutations, and have differentiated these stem cell models into motor neurons which enabled us to conduct transcriptomic and biochemical studies. In addition, we have utilized antisense-oligonucleotides (ASOs) from ISIS Pharmaceuticals to deplete mutant C9ORF72 in motor neurons. We expect our efforts to provide mechanistic insights and a potential therapy in human cells.