We thank J. Ravits and C. Lagier-Tourenne (University of California, San Diego) for sharing the control and C9orf72 ALS patient fibroblasts used in this study. We acknowledge microscopy assistance from the Stanford Neuroscience Microscopy Service, supported by US National Institutes of Health (NIH) grant NS069375. This work was supported by NIH grants 1R01NS065317 and 1R01NS073660 (A.D.G.). A.D.G. is supported by the Packard Center for ALS Research at Johns Hopkins and Target ALS. A.J. is supported by the Dean's Postdoctoral Fellowship from Stanford University. S.S. is a recipient of NIH K99/R00 Award (NS091538) and Target ALS Springboard Fellowship. J.M. and F.H.G. are supported by JPB Foundation, the Helmsley Foundation and the Mathers Foundation. Additional research funding was provided by the KU Leuven, the European Research Council in the context of the European's Seventh Framework Programme (FP7/2007-2013 and ERC grant agreement 340429), the Fund for Scientific Research Flanders (FWO-Vlaanderen) G.0983.14N, the Interuniversity Attraction Poles Programme P7/16 initiated by the Belgian Science Policy Office, the Association Belge contre les Maladies Neuro-Musculaires (ABMM), the ALS Liga (Belgium) and the Opening the Future fund. S.B. received a fellowship from the Agency for Innovation by Science and Technology IWT. E.B. holds a postdoctoral fellowship from FWO-Vlaanderen. W.R. is supported through the E. von Behring Chair for Neuromuscular and Neurodegenerative Disorders.