In the N-terminus of MUC-1, there is a large region of tandem repeat units (12 consecutive repeats of ~20 amino acids (HGVTSAPDTRPAPGSTAPPA), spanning H136 to S380 in this form of the protein) which convolutes the analysis of this glycoprotein. The UVPD spectrum of an exemplative glycopeptide with a fucosylated core 2 glycan and a core 1 glycan, (acetyl)APDT[+876.32]RPAPGS[+365.13] (2+, m/z 1126.48), is shown in Figure . An extensive array of fragment ions is produced, including two products from cross-ring cleavages that confirm the positions of the Gal and Fuc as the third carbon and fourth carbon atoms of GlcNAc, respectively. One of the cross-ring cleavage products is A2 (m/z 221.10) that encompasses the Fuc and fourth, fifth, and sixth carbons of GlcNAc. The second is 3,5 X2 2+ (m/z 1016.42) that contains the peptide and second glycan at S10 but has lost the fourth, fifth, and sixth carbons of GlcNAc. Together, these two ions piece together the fucosylated glycan at T4 and reveal the peptide sequence, including the N-terminal acetylation. Additional glycosidic fragments confirm details about the structure of the fucosylated glycan, such as the glycan-only fragments consisting of GlcNAcFuc, with or repeats located between the two heavily glycosylated regions show significantly less glycosylation, which might partly be due to the accessibility of those particular putative glycosites and their ambiguity as repeats within the protein sequence. However, because different glycosites and glycans are identified for the glycopeptides originating from the 20 residue repeats, it suggests there is true site-specific diversity among the 13 consecutive repeats. Simple core 1 glycans were detected throughout the tandem repeats with the greatest frequency between the regions of highest glycodiversity (S130-S180 and S360-T371). Despite the lack of heavy glycosylation through the tandem repeats of MUC-1, the fact that significant glycodiversity was uncovered using the SmE digestion/UVPD strategy suggests that this is a promising approach.