Amyotrophic lateral sclerosis (ALS) is a chronic progressive neurodegenerative disease that remains incurable, with limited existing therapies or drugs available. Familial ALS can be caused by mutations in various genes. In Asia, mutations in the FUS gene are relatively common among early-onset familial ALS patients. Reducing the levels of toxic FUS protein may be an effective therapeutic approach for such ALS patients without causing side effects. RAG-21 is a small interfering ribonucleic acid (siRNA) with a molecular weight of 20 kDa. Through the RNA interference mechanism, it targets the FUS gene, recognizes the corresponding mRNA, and mediates its degradation, thereby downregulating FUS gene expression and reducing toxic FUS protein levels. Accordingly, this project plans to conduct a single-center, dose-escalation clinical study aimed at evaluating the safety, tolerability, and pharmacokinetics of intrathecal bolus administration of RAG-21 in ALS patients carrying FUS gene mutations.