O-linked glycosylation is a type of post-translational modification where sugar molecules are attached to the oxygen atom of serine or threonine residues in proteins. This process plays a crucial role in various biological functions, including protein stability, cell signaling, and immune responses. The sugars added during O-linked glycosylation can vary, leading to diverse structures that influence how proteins interact with other molecules. This modification is particularly important in mucins, which are glycoproteins found in mucus, and in the functioning of glycoproteins involved in cell communication and adhesion.