B3GAT3 (Beta-1,3-Glucuronyltransferase 3), also known as Glucuronosyltransferase I (GlcAT-I), is an enzyme encoded by the B3GAT3 gene. This enzyme belongs to the glycosyltransferase family and catalyzes the transfer of glucuronic acid (GlcA) from UDP-GlcA to the terminal galactose residue of the linkage region tetrasaccharide (GlcA-Gal-Gal-Xyl-) during the biosynthesis of proteoglycans. The addition of glucuronic acid is a critical step in forming the glycosaminoglycan-protein linkage region, which serves as the attachment site for heparan sulfate and chondroitin sulfate chains. Mutations in the B3GAT3 gene cause a rare autosomal recessive disorder known as linkeropathy or B3GAT3-related multiple joint dislocations.