A 4-nitrophenyl β-glycoside of N,N′-diacetylchitobiose (GlcNAcβ1-4GlcNAc), the conserved core disaccharide of N-linked glycans. Used as a chromogenic substrate for chitinases, endo-β-N-acetylglucosaminidases (e.g., Endo H, Endo F), and β-N-acetylhexosaminidases; enzymatic cleavage releases 4-nitrophenol. 4-Nitrophenyl (pNP) glycosides are chromogenic enzyme substrates: cleavage of the glycosidic bond by the target enzyme releases free 4-nitrophenol, which is yellow under basic conditions (absorbance maximum ~405 nm), enabling straightforward spectrophotometric assays of enzyme activity in solution and in high-throughput screening formats.