Uridine diphosphate N-acetylgalactosamine or UDP-GalNAc is a nucleotide sugar composed of uridine diphosphate (UDP) and N-acetyl galactosamine (GalNAc). It is used by glycosyltransferases to transfer N-acetylgalactosamine residues to substrates. UDP-GalNAc is an important building block for the production of glycoproteins and glycolipids in the body. It also serves as a precursor for the synthesis of mucin-type O-glycans, which are important components of mucus and play important roles in biological processes such as cell signaling, immune defense, and lubrication of the digestive tract.