PLP1 and Pelizeaus-Merzbacher spectrum disorder: However, PMD patients who have PLP1 mutations that cause greatly reduced amounts of PLP, but not DM20, or mutant mice expressing DM20, but not PLP, show a slow disintegration of the compact layers of the myelin sheath with aging, which also suggests a role for the intracellular loop of PLP in stabilizing compact myelin [8–10].