Beta-1,3-N-acetylglucosaminyltransferase that synthesizes the core 3 structure of the O-glycan, an important precursor in the biosynthesis of mucin-type glycoproteins. Plays an important role in the synthesis of mucin-type O-glycans in digestive organs. 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-[N-acetyl-beta-D-glucosaminyl-(1->3)-N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + H(+) + UDP 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-seryl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-[N-acetyl-beta-D-glucosaminyl-(1->3)-N-acetyl-alpha-D-galactosaminyl]-L-seryl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Present in stomach and colon (at protein level). Restricted in the stomach, colon and small intestine, where core 3 structure is present. Down-regulated in gastric and colorectal carcinomas, suggesting that it may be used as a marker for distinguishing between benign adenomas and premalignant lesions (at protein level). Injection into nude mice significantly suppress lung metastasis, indicating that the core structures of O-glycans are profoundly involved in the metastatic capacity of cancer cells. Belongs to the glycosyltransferase 31 family.