X-linked spastic paraplegia and Pelizaeus-Merzbacher disease are allelic disorders at the proteolipid protein locus.

Three forms of X-linked spastic paraplegia (SPG) have been defined.
One locus (SPG 1) maps to Xq28 while two clinically distinct forms map to Xq22 (SPG2). A rare X-linked dysmyelinating disorder of the central nervous system, Pelizaeus-Merzbacher disease (PMD), has also been mapped to Xq21-q22, and is caused by mutations in the proteolipid protein gene (PLP) which encodes two myelin proteins, PLP and DM20. While narrowing the genetic interval containing SPG2 in a large pedigree, we found that PLP was the closest marker to the disease locus, implicating PLP as a possible candidate gene.
We have found that a point mutation (His139Tyr) in exon 3B of an affected male produces a mutant PLP but a normal DM20, and segregates with the disease (Zmax = 6. 63, theta = 0. 00). It appears, therefore, that SPG2 and PMD are allelic disorders 
