This is the published version. It first appeared at http://onlinelibrary.wiley.com/doi/10.1002/anie.201502813/abstract. ; The structural features of MUC1-like glycopeptides bearing the Tn antigen (?-O-GalNAc-Ser/Thr) in complex with an anti MUC-1 antibody are reported at atomic resolution. For the ?-O-GalNAc-Ser derivative, the glycosidic linkage adopts a high-energy conformation, barely populated in the free state. This unusual structure (also observed in an ?-S-GalNAc-Cys mimic) is stabilized by hydrogen bonds between the peptidic fragment and the sugar. The selection of a particular peptide structure by the antibody is thus propagated to the carbohydrate through carbohydrate/peptide contacts, which force a change in the orientation of the sugar moiety. This seems to be unfeasible in the ?-O-GalNAc-Thr glycopeptide owing to the more limited flexibility of the side chain imposed by the methyl group. Our data demonstrate the non-equivalence of Ser and Thr O-glycosylation points in molecular recognition processes. These features provide insight into the occurrence in nature of the APDTRP epitope for anti-MUC1 antibodies. ; We thank the Ministerio de Econom a y Competitividad/FEDER (project CTQ2012-36365, CTQ2012-32065, BFU2010-19504, CTQ2013-44367-C2-2-P, UNLR13-4E-1931 and grant I.C.) and DGA (B89) for financial support. N.M.-S. and D.M. thank Universidad de La Rioja for FPI grants. We thank Katherine Stott (Department of Biochemistry, Cambridge University) for technical help with the BLI experiments. G.J.L.B. thanks financial support from the EPSRC. G.J.L.B. is a Royal Society University Research Fellow. M.S. thanks the Generalitat de Catalunya and Universitat Rovira i Virgili for financial support.