The function of GCNT3 (glucosaminyl (N-acetyl) transferase 3, mucin type, ENSG00000140297) is as follows. Glycosyltransferase that can synthesize all known mucin beta 6 N-acetylglucosaminides. Mediates core 2 and core 4 O-glycan branching, 2 important steps in mucin-type biosynthesis. Also has I- branching enzyme activity by converting linear into branched poly-N- acetyllactosaminoglycans, leading to introduce the blood group I antigen during embryonic development. .