Glycosyltransferase involved in the elongation of O-linked ligands to activate Notch signaling. Possesses fucose-specific beta-1,3-N-acetylglucosaminyltransferase activity; extends the O-linked fucose on the Notch EGF repeats. Boundary-specific cell-signaling molecule that is responsible for dorsal-ventral cell interactions during wing development. 3-O-(alpha-L-fucosyl)-L-threonyl-[EGF-like domain protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl-(1->3)-alpha-L-fucosyl)-L-threonyl-[EGF-like domain protein] + H(+) + UDP 3-O-(alpha-L-fucosyl)-L-seryl-[EGF-like domain protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl-(1->3)-alpha-L-fucosyl)-L-seryl-[EGF-like domain protein] + H(+) + UDP Expressed in dorsal cells. Belongs to the glycosyltransferase 31 family.