All of our genes code for specific proteins, which have specific roles in our bodies. The PLP1 gene provides the code to build proteolipid protein 1, and a similar protein called DM20. These are the main proteins which are found in myelin. If the PLP1 gene contains a mutation, these proteins will not be made. If these proteins are not made then the myelin sheath cannot be formed correctly. This leads to the symptoms of Pelizaeus-Merzbacher disease.