Glycosylation site refers to a specific position or region on a protein molecule where one or more amino acid residues can bind to sugar molecules through valent bonds. Specific to a certain protein, a glycosylation site refers to a specific position on the protein's amino acid chain that can bind to sugar molecules. For example, in N-glycosylation, an N-acetyl aminoglycosyl group is usually attached to the amino group of the amino acid aspartate or aspartic acid. In O-glycosylation, a sugar is usually attached to the hydroxyl group of lysine.