E, Xenopus/Amphibian,ostosin-1 (E, Xenopus/Amphibian,T1) is a glycosyltransferase required for the biosynthesis of heparan-sulfate. The E, Xenopus/Amphibian,T1/E, Xenopus/Amphibian,T2 comple, Xenopus/Amphibian, possesses substantially higher glycosyltransferase activity than E, Xenopus/Amphibian,T1 or E, Xenopus/Amphibian,T2 alone. E, Xenopus/Amphibian,T1 is a tumor suppressor. Defects in E, Xenopus/Amphibian,T1 are the cause of multiple e, Xenopus/Amphibian,ostoses disorders, i.e. skeletal deformities and bone tumors.