Treatment with a cell-penetrating peptide that prevented nuclear export of unprocessed C9ORF72 RNA and its subsequent translation into neurotoxic dipeptide repeat proteins reduced motor neuron damage and death both in fruit fly models of amyotrophic lateral sclerosis (ALS), and in patient-derived induced neuronal precursor cells (iNPCs). The work suggests that targeting nuclear export could be a therapeutic option in ALS, and possibly also frontotemporal dementia (FTD), where C9ORF72 mutations also play a role.