Predominantly fucosylates the innermost N-acetyl glucosamine (GlcNAc) residue in biantennary N-glycan acceptors. Postulated to generate core alpha(1->3)-fucose epitope within the chitobiose unit of biantennary N-glycans, providing for a recognition signal to reorient aberrantly folded glycoproteins for degradation. Involved in biosynthesis of Lewis X-carrying biantennary N-glycans that regulate neuron stem cell self-renewal during brain development 