Graphic diagram of GAG core tetrasaccharide biosynthesis and HS chain polymerization. The Ser/Thr-containing polypeptide core protein is glycosylated and modified through ten enzymatic reactions occurring in the endoplasmic reticulum and Golgi. Numbered steps correspond to the reactions catalyzed by enzymes encoded by the following genes (1): XYLT1, XYLT2, (2): B4GALT7, (3): B3GALT6, (4): B3GALT3, (5): EXTL2, EXTL3, (6): EXT1, EXT2, (7): EXT1, EXT2, EXTL1, EXTL3. (8): CSGALNACT1, CSGALNACT2, (9): CHSY1, CHPF1, CHSY3, CHPF2, and (10): DSE1, DSE2.