preferred_name: 3-O-[alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + UDP-alpha-D-xylose <=> 3-O-[alpha-D-xylosyl-(1->3)-alpha-D-xylosyl-(1->3)-beta-D-glucosyl]-L-seryl-[EGF-like domain protein] + UDP + H(+)