This glycopeptide is an N-acetyl galactosamine (GalNAc)-modified MUC5AC mucin peptide containing the single site of threonine 13 labeled with GalNAc (T*). Polypeptide N-acetylgalactosaminyltransferase (ppGaNTase) catalyzes the transfer of GalNAc from the nucleotide sugar UDP-GalNAc to threonine. The MUC5AC gene is mainly expressed in gastric and tracheo-bronchial mucosae, and some tumors.