Genomic rearrangements leading to the duplication of the X-linked proteolipid protein 1 (PLP1) gene are the major mutational cause for PMD and explain ~80% of patients; point mutations in PLP1 occur less frequently, and higher copy number gains (e. g. triplications) and deletions are rare [14–17]. The gene discussed is PLP1; the disease is Pelizeaus-Merzbacher spectrum disorder.