These results provide compelling evidence that the expression level of the CRISPR/Cas13d system is a critical determinant of its ability to effectively inhibit C9orf72 RAN translation, highlighting the importance of optimizing Cas13d expression levels to maximize the therapeutic potential of this approach for treating C9orf72-ALS/FTD. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.