The mean of PLP/β2 ratios was 19 ± 13.8 in the control group (minimum 0.36, maximum 39.9) and 80.05 ± 21.7 (minimum 56.5, maximum 120.5) in the PMD group, demonstrating a significant PLP‐DM20 mRNA overexpression in the nerves from PMD patients with a PLP1 duplication. Here, PLP1 is linked to Pelizeaus-Merzbacher spectrum disorder.