Provided herein are CasX:gNA systems comprising CasX polypeptides, guide nucleic acids (gNA), and optionally donor template nucleic acids useful in the modification of a SOD1 gene. The systems are also useful for introduction into cells, for example eukaryotic cells having mutations in the SOD1 protein or the SOD1 regulatory element. Also provided are methods of using such CasX:gNA systems to modify cells having such mutations and utility in methods of treatment of a subject with a SOD1-related disease.