An Fmoc-protected L-serine building block carrying a GlcNAcβ1-3GalNAcα disaccharide, designed for solid-phase peptide synthesis (SPPS) of glycopeptides. The Nα-Fmoc group is removed under standard base conditions (piperidine) during chain elongation, while the free α-carboxylic acid is available for coupling to the growing resin-bound peptide. The sugar hydroxyl groups are protected as acetate esters (peracetate), which keep the glycan inert during synthesis and are removed under mild base (e.g., sodium methoxide / hydrazine) after assembly. The installed glycan is the core 3 structure (GlcNAcβ1-3GalNAcα-O-Ser/Thr), a mucin-type O-glycan core found in normal epithelial tissue and downregulated in many carcinomas, used in O-glycobiology and cancer research.