A monosaccharide glucose derivative significant in several biological systems. Bacterial cell walls consist of structural units of alternating of GlcNAc and N-acetylmuramic acid (MurNAc) subunits cross-linked with oligopeptides. Chitin, the outer coverings of insects and crustaceans, consists of GlcNAc polymers. It also functions as an atypical neurotransmitter in pain transmission.