PLP1 and Pelizeaus-Merzbacher spectrum disorder: The essential role played by the myelin proteolipid protein (PLP) in normal functioning of the central nervous system (CNS) is illustrated by the fact that mutations in PLP1 result in either Pelizaeus-Merzbacher disease (PMD) or spastic paraplegia type 2 (SPG2), which are X-linked leukodystrophies (Wolf et al., 1999; Inoue, 2005; Garbern, 2007).