Exostosin1 and 2 (EXT1 and 2) are dual specificity enzymes which catalyze the addition of N acetylglucosamine (GlcNAc) and glucuronate (GlcA) to extend the GAG chain on the protein linker sequence. Heparan is synthesized once GlcNAc is transferred to this sequence. EXT1 and 2 form a heterodimer which translocates to the Golgi apparatus from the ER membrane (McCormick et al. 2000). Defects in EXT1 or 2 cause the hereditary bone disorders multiple exostoses type 1 (MIM:133700) and 2 (MIM:133701) (Wuyts et al. 1998, Bernard et al. 2001).