Rare null mutations, such as gene deletions or nonsense/frame shift mutations that result in premature terminations (presumably degraded by nonsense mediated mRNA decay) leading to no PLP1 production can cause a mild but slowly progressive PMD phenotype (Inoue et al., 2002; Garbern, 2007). The gene discussed is PLP1; the disease is Pelizeaus-Merzbacher spectrum disorder.