Pathways of O-glycosylation on the ocular surface epithelia. Glycosyltransferases potentially involved in the biosynthesis of mucin-type O-glycans present on tear mucins were identified using a public microarray database, as described in Materials and Methods. These included thirteen polypeptide GalNAc-transferases (GALNT) involved in the initiation of mucin-type O-glycosylation, the core 1 β-3-galactosyltransferase (T-synthase) involved in the addition of galactose to the Tn antigen, three α2-6-sialyltransferases (ST6GalNAc), and two α2-3-sialyltransferases (ST3Gal). Core 2 (dotted line) in conjunctival epithelial cells7 is synthesized by core 2 β1,6 N-acetylglucosaminyltransferase (C2GnT-M). The relative levels of expression of individual glycosyltransferase isoforms are indicated in parenthesis. Symbols represent galactose (●), N-acetylgalactosamine (□), N-acetylglucosamine (■), and N-acetylneuraminic acid (◆).