The biosynthesis of dermatan sulfate/chondroitin sulfate and heparin/heparan sulfate glycosaminoglycans (GAGs) starts with the formation of a tetrasaccharide linker sequence to the core protein. The first step is the addition of xylose to the hydroxy group of specific serine residues on the core protein. Subsequent additions of two galactoses and a glucuronide moiety completes the linker sequence. From here, the next hexosamine addition is critical as it determines which GAG is formed (Lamberg & Stoolmiller 1974, Pavao et al. 2006).