Mutations in the EXT1 and EXT2 genes cause MHE. They code the instructions to make exostosin-1 and exostosin-2, which later bind together and form a complex structure in the golgi apparatus, thus damaging haparan sulfate. Without correct cumbers of haparan sulfate, MHE occurs. It is not known why.
Chemical Composition of haparan sulfate
10% of cases of MHE are not caused by the EXT genes. It is not known what causes these cases.