AbstractTyrosine sulfation, a post-translational modification, can enhance and often determine protein-protein interaction specificity. Sulfotyrosyl residues (sTyr) are formed by tyrosyl-protein sulfotransferase during maturation in the golgi apparatus, and most often occur singly or as a cluster of two or three sTyr within a six-residue span. With both negative charge and aromatic character, sTyr enables numerous atomic contacts as visualized in binding interface structural models, and so there is no discernible binding site consensus. Found exclusively in secreted proteins, sTyr residues occur in four broad sequence contexts. First, a single sTyr residue is critical for diverse high-affinity interactions between peptide hormones and their receptor in both plants and animals. Second, sTyr clusters within structurally flexible anionic segments are essential for a variety of processes including coreceptor binding to the HIV-1 envelope spike protein during virus entry, chemokine interactions with many chemokine receptors, and leukocyte rolling cell adhesion. Third, a subcategory of sTyr clusters occurs in the context of conserved acidic sequences termed hirudin-like motifs that enable several proteins to interact with thrombin, central to normal blood-clotting. Consequently, many proven and potential therapeutic proteins derived from blood-consuming invertebrates depend on sTyr residues for their activity. Fourth, a few proteins that interact with collagen or other proteins contain one or more sTyr residues within an acidic residue array. Refined methods to direct sTyr incorporation in peptides synthesized both in vitro and in vivo, together with continued advances in MS and affinity detection, promise to accelerate discoveries of sTyr occurrence and function.