We report a third mutation of the proteolipid protein gene in male Japanese patients with X-linked spastic paraplegia. Although the proteolipid protein gene encodes two myelin proteins, proteolipid protein and DM 20, our W144X mutation resides in the latter part of exon 3 (exon 3B), which is spliced out in DM 20. This mutation may reserve the function of DM 20. Findings in our patients support that this form of spastic paraplesia is allelic to Pelizaeus-Merzbacher disease and that the mild clinical phenotype of this disorder may be related to a mutation within exon 3B of the PLP gene.