The purpose of the study is to prospectively assess longitudinal changes in proteolipid protein 1 (PLP1) protein, disease-related biomarkers in cerebral spinal fluid (CSF) and blood, neuroimaging parameters relevant to Pelizaeus-Merzbacher disease (PMD) and longitudinal changes in performance on clinical, participant, and caregiver-reported outcome assessments to inform the development of therapies for PMD.