The O-glycosylation of serine and threonine residues with glycans linked through GalNAc is a common post-translational modification of a wide range of secreted and membrane-associated proteins. One class of highly O-glycosylated proteins are the mucus glycoproteins commonly called mucins (1). These glycoproteins are typically 20 -30% Ser and Thr, are between 50 and 80% carbohydrate by weight, and commonly contain tandemly repeated peptide sequences. Mucins and glycoproteins containing mucin-like domains play a major role protecting epithelial cell surfaces and are thought to be involved in modulating many biological processes including the immune response, adhesion, inflammation, tumorigenesis, and perhaps development (2-9).The first step in O-glycan synthesis is the transfer of GalNAc to Ser/Thr residues, by UDP-GalNAc:polypeptide ␣-GalNAc transferase (ppGalNAc transferase).1 To date, 10 ppGalNAc transferase isoforms have been described (9 -16). Although each transferase has not been fully characterized, their peptide substrate specificities vary within the family (11, 17-21); many show sensitivity to prior glycosylation, and others require prior addition of GalNAc for activity (9,14,20,(22)(23)(24). The expression of ppGalNAc transferase isoforms with different peptide and/or glycopeptide specificities therefore represents the first step in the regulation of O-glycan structure by peptide sequence in vivo. Subsequent elongation of O-linked glycans proceeds by the stepwise addition of single sugar residues via a series of substrate-specific Golgi resident transferases (25). It is well accepted that Golgi localization, nucleotide sugar concentration, and competition among transferases are the major dictates of O-glycan structure and elongation (25-31). Depending on the initial and subsequent substitutions on the GalNAc residue, a wide range of O-linked core structures is possible (25). Little, however, is known of the effects of local peptide sequence on O-glycan elongation (23, 32-37), although a number of examples of site-specific elongation of N-linked glycans are known (30, 38 -46). Factors affecting site-specific N-glycan