The mean of PLP/β2 ratios was 2.69 ± 1.31 (minimum 0.85, maximum 5.25) in the control group and 33.15 ± 32.18 (minimum 6.54, maximum 102.33) in the PMD group demonstrating a significant PLP‐DM20 mRNA overexpression in the fibroblasts from PMD patients with PLP1 duplications. This evidence concerns the gene PLP1 and Pelizeaus-Merzbacher spectrum disorder.