Duplications of the PLP1 locus represent the most common cause of PMD (Sistermans et al., 1998; Mimault et al., 1999); however, variation in the size and break points of the duplicated region on the X chromosome and in the site of recombination likely contribute to the range of severity, even within this genetic subtype (Inoue et al., 1999; Hodes et al., 2000). Here, PLP1 is linked to Pelizeaus-Merzbacher spectrum disorder.