preferred_name: a 3-O-{alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-alpha-D-GalNAc}-L-seryl-[protein] + UDP-N-acetyl-alpha-D-galactosamine <=> a 3-O-{[alpha-Neu5Ac-(2->3)]-beta-D-GalNAc-(1->4)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-alpha-D-GalNAc}-L-seryl-[protein] + UDP + H(+)