Inclusion Criteria:  ALS-specific: Diagnosis of ALS based on clinical manifestations. ALS-specific: Clinically diagnosed possible, laboratory supported probable, probable, or definite criteria for diagnosing ALS according to the World Federation of Neurology revised El Escorial criteria. ALS-specific: Patients receiving riluzole have been on a stable dose for a minimum of 30 days. ALS-specific: Patients on edaravone have received a minimum of 1 cycle (28 days). ALS-specific: Patients discontinuing riluzole or edaravone had the last dose administered ≥1 month prior to Screening. FTD-specific: Must have Global Clinical Dementia Rating - Frontotemporal Lobar Degeneration (CDR® plus NACC FTLD) score of 0.5 or 1. FTD-specific: Able to undergo periodic magnetic resonance imaging (MRI) of the brain. Participants with mixed phenotype (ALS and FTD) need not undergo MRI if their ALS symptoms prevent it. Mixed-phenotype: Patients who are mixed phenotype (ALS and FTD) must meet both the ALS-specific and FTD-specific criteria.  Exclusion Criteria:  Clinically significant medical finding on the physical examination other than C9orf72-associated ALS or FTD that, in the judgment of the Investigator, will make the patient unsuitable for participation in, and/or completion of the trial procedures Received any other investigational drug, biological agent, or device within 1 month or 5 half-lives of study agent, whichever is longer. Received an investigational oligonucleotide, within the past 6 months or 5 half-lives of the drug, whichever is longer.