N-Acetylgalactosamine, also known as GalNAc, belongs to the class of organic compounds known as N-acyl-alpha-hexosamines. These are carbohydrate derivatives containing a hexose moiety in which the oxygen atom is replaced by an N-acyl group. N-Acetylgalactosamine is also classified as an amino sugar derivative of galactose. In humans GalNAc functions as the terminal carbohydrate forming the antigen of blood group A. GalNAc is typically the first monosaccharide that connects serine or threonine during protein O-glycosylation and the formation of glycoproteins. This is often referred to as mucin-type O-glycosylation, as the mucins (a class of a family of high molecular weight, heavily glycosylated proteins produced by epithelial tissues in most animals which have an ability to form gels) are heavily O-GalNAc modified. Interestingly, mammals have genes encoding for approximately 20 different polypeptide-N-acetylgalactosaminyltransferases (ppGalNAcTs), all of which transfer GalNAc from UDP-GalNAc to a hydroxyl-containing amino acids such as serine or threonine. N- O-GalNAc-containing glycoproteins appear to play a variety of essential roles. Among these is the ability of the mucins to hydrate and protect tissues by trapping bacteria. These O-glycans can also significantly alter the conformation of the protein and on the heavily modified proteins may protect the polypeptide from proteolytic digestion. O-GalNAc structures also appear to play an essential role in sperm‚Äìegg interactions. From a pathophysiological perspective, O-GalNAc modification appears to play a critical role in the immune system, cell‚Äìcell interactions, and cancer.  N-Acetylgalactosamine is an important constituent of brain heteropolysaccharides (glycoproteins). The concentration of the N-acetylgalactosamine-containing glycoproteins in the 3-year-old cerebral gray matter from human brain is 7-15 times greater than in 8-year old tissue and 15-30 times greater than in 72-year-old tissue. Outside of the human body, N-Acetylgalactosamine has been detected, but not quantified in, several different foods, such as prickly pears, italian sweet red peppers, wheats, silver lindens, and sour cherries. This could make N-acetylgalactosamine a potential biomarker for the consumption of these foods.