PLP1 and Pelizeaus-Merzbacher spectrum disorder: Pelizaeus-Merzbacher disease (PMD), the prototypic hypomyelinating leukodystrophy, is caused by mutations in the highly conserved, X-linked PLP1 gene (Gencic et al., 1989; Boespflug-Tanguy et al., 1994; Garbern et al., 1999), which encodes proteolipid protein (PLP) and its isoform DM20 (Nave et al., 1987).