4-Azidobutyric acid NHS ester is an amine-reactive heterobifunctional cross-linker. At neutral or slightly basic pH (7-9), it rapidly attacks solvent accessible lysine-NH*2* groups, N-termini and amines and installs the alkylazido group that serves as a bioorthogonal handle for copper-catalyzed alkyne-azide cycloaddition (CuAAC).