preferred_name: 3-O-[N-acetyl-beta-D-glucosaminyl-(1->2)-alpha-D-mannosyl]-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine <=> O(3)-{N-acetyl-beta-D-glucosaminyl-(1->2)-[N-acetyl-beta-D-glucosaminyl-(1->6)]-alpha-D-mannosyl}-L-threonyl-[protein] + UDP + H(+)