Fmoc-NH-Azide-PEG4-L-Lysine-PFP ester serves as a cleavable 4-unit PEG-based ADC linker in the formulation of antibody-drug conjugates (ADCs) [1]. This compound acts as a click chemistry agent featuring an Azide group, enabling it to engage in copper-catalyzed azide-alkyne cycloaddition reactions (CuAAc) with alkyne-bearing molecules. Additionally, it facilitates strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules equipped with DBCO or BCN groups.