N3-PEG4-amido-Lys(Fmoc)-acid is a 4 unit PEG ADC linker utilized in the production of antibody-drug conjugates (ADCs) [1]. This compound, functioning as a click chemistry reagent, is equipped with an Azide group that enables it to partake in copper-catalyzed azide-alkyne cycloaddition (CuAAc) with Alkyne group-bearing molecules. Additionally, it is capable of engaging in strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules containing DBCO or BCN groups.