Azido-modified analogue of UDP-GalNAc; serves as a substrate for N-acetylgalactosaminyltransferases (ppGalNAc-Ts) enabling metabolic and chemoenzymatic glycan labeling; the azidoacetyl handle permits bioorthogonal conjugation via copper-catalyzed azide–alkyne cycloaddition (CuAAC) and strain-promoted azide–alkyne cycloaddition (SPAAC); widely used in O-GalNAc glycoproteomics, mucin-type glycan imaging, and site-specific antibody–drug conjugate (ADC) construction