The Core-4 trisaccharide, GlcNAcβ1-3(GlcNAcβ1-6)GalNAc, is one of the common O-linked glycan cores. These building blocks can be used as ligands of glycan binding proteins, standards or for the precise synthesis of glycopeptides containing the Core-4 trisaccharide. We offer a variety of Core-4 modified glycoamino acids.