This glycopeptide is a MUC5AC peptide with two sites (Thr3 and Thr13) labeled with GalNAc, where the asterisk denotes GalNAc-modified residues. Mucin type O-linked glycosylation is initiated by the action of a family of UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferases (ppGaNTase), which catalyze the transfer of GalNAc from the nucleotide sugar UDP-GalNAc to the hydroxyl group of either serine or threonine.