An N-acetyl, C-terminal methyl ester L-serine glycoamino acid carrying a Galβ1-3GalNAcα disaccharide (T antigen). Both termini are capped, making this a self-contained model compound for conformational, analytical, or enzymatic studies rather than a chain-extendable SPPS building block. The installed glycan is the T antigen / core 1 structure (Galβ1-3GalNAcα-O-Ser/Thr), a tumor-associated carbohydrate antigen and ligand for galectins and the macrophage galactose lectin, used to build glycopeptides for cancer immunology and O-glycobiology research.