Orbit Study: A Study to Evaluate the Safety, Pharmacokinetics, and Pharmacodynamics of Intrathecally Administered ION356 in Participants With Pelizaeus Merzbacher Disease (PMD)

Key Inclusion Criteria 1. Participant's parent or legally accepted representative can provide informed consent, attend all scheduled study visits, provide feedback regarding the participant's symptoms, and can comply with all study requirements. 2. Diagnosis of PMD with genetic confirmation of PLP1 gene duplication. 3. Clinical phenotype and brain imaging consistent with a diagnosis of PMD. 4. Male between the ages of 2 and 17 years, inclusive, at the time of informed consent. 5. Able and willing to meet all study requirements (in the opinion of the Investigator), including travel to Study Center, procedures, measurements, and visits. Exclusion Criteria 1. Clinically significant abnormalities in medical history, laboratory tests or physical examination. 2. Unwillingness to comply with study procedures, including follow-up, as specified by this protocol, or unwillingness to cooperate fully with the Investigator. 3. Any contraindication or unwillingness to undergo magnetic resonance imaging (MRI). 4. Treatment with another investigational drug, biological agent, or device within 1 month of Screening, or 5 half-lives of the investigational agent, whichever is longer. 5. Previous treatment with an oligonucleotide (including small interfering ribonucleic acid) within 4 months of Screening if a single dose was received, or within 12 months of Screening if multiple doses were received. This exclusion does not apply to vaccines (both messenger ribonucleic acid \[mRNA\] and viral vector vaccines). 6. History of gene therapy or cell transplantation, or any experimental brain surgery. 7. Current obstructive hydrocephalus. 8. Known brain or spinal disease or previous spinal surgery that would interfere with the lumbar puncture (LP) process, CSF circulation, or safety assessment. 9. Hospitalization for any major medical or surgical procedure involving general anesthesia within 12 weeks prior to Screening or planned during the study. 10. Have any other conditions, which, in the opinion of the Investigator, would make the participant unsuitable for inclusion, or could interfere with the participant participating in or completing the study.