The C9ORF72 hexanucleotide repeat expansion (HRE) is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). This study shows that the HRE RNA disrupts nucleocytoplasmic transport, leading to neurodegeneration. The HRE RNA binds to RanGAP1, a key regulator of nucleocytoplasmic transport, and mislocalizes it in Drosophila and human cells. This mislocalization impairs nuclear import and nuclear export, contributing to neurotoxicity. Enhancing nuclear import or suppressing nuclear export of proteins suppresses neurodegeneration. The HRE RNA forms G-quadruplex structures that bind and sequester RNA-binding proteins, leading to impaired nucleocytoplasmic transport. Small molecules and antisense oligonucleotides targeting the HRE G-quadruplexes rescue the transport defects. The study also shows that RanGAP1 is mislocalized in C9ORF72 ALS patient-derived induced pluripotent stem cells (iPSNs) and brain tissue. Nucleocytoplasmic transport defects may be a fundamental pathway for ALS and FTD amenable to pharmacotherapeutic intervention. The study demonstrates that RanGAP1 is a potent suppressor of C9ORF72 HRE-mediated neurodegeneration in Drosophila and human cells. Modulating nucleocytoplasmic transport presents a potential therapeutic strategy for neurodegenerative diseases characterized by the C9ORF72 HRE. The study also shows that the HRE RNA disrupts the nucleocytoplasmic Ran gradient, leading to impaired nuclear import of proteins containing a classical nuclear localization signal (NLS). Pharmacologic rescue of HRE-mediated neurodegeneration is achieved by targeting the HRE RNA with antisense oligonucleotides or by inhibiting the G-quadruplex structure with a porphyrin compound. These findings suggest that nucleocytoplasmic transport defects are a fundamental mechanism for C9ORF72-related ALS and FTD.The C9ORF72 hexanucleotide repeat expansion (HRE) is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). This study shows that the HRE RNA disrupts nucleocytoplasmic transport, leading to neurodegeneration. The HRE RNA binds to RanGAP1, a key regulator of nucleocytoplasmic transport, and mislocalizes it in Drosophila and human cells. This mislocalization impairs nuclear import and nuclear export, contributing to neurotoxicity. Enhancing nuclear import or suppressing nuclear export of proteins suppresses neurodegeneration. The HRE RNA forms G-quadruplex structures that bind and sequester RNA-binding proteins, leading to impaired nucleocytoplasmic transport. Small molecules and antisense oligonucleotides targeting the HRE G-quadruplexes rescue the transport defects. The study also shows that RanGAP1 is mislocalized in C9ORF72 ALS patient-derived induced pluripotent stem cells (iPSNs) and brain tissue. Nucleocytoplasmic transport defects may be a fundamental pathway for ALS and FTD amenable to pharmacotherapeutic intervention. The study demonstrates that RanGAP1 is a potent suppressor of C9ORF72 HRE-mediated neurodegeneration in Drosophila and human cells. Modulating nucleocytoplasmic transport presents a potential therapeutic strategy for neurodegenerative diseases characterized by the C9ORF72 HRE. The study also shows that the HRE RNA disrupts the nucleocytoplasmic Ran gradient, leading to impaired nuclear import of proteins containing a classical nuclear localization signal (NLS). Pharmacologic rescue of HRE-mediated neurodegeneration is achieved by targeting the HRE RNA with antisense oligonucleotides or by inhibiting the G-quadruplex structure with a porphyrin compound. These findings suggest that nucleocytoplasmic transport defects are a fundamental mechanism for C9ORF72-related ALS and FTD.