Azido-PEG4-formylhydrazine-Boc is a PROTAC linker within the PEG category, suitable for PROTAC molecule synthesis. This compound serves as a click chemistry reagent, featuring an azide group that can participate in copper-catalyzed azide-alkyne cycloaddition reactions (CuAAC) with alkyne-containing molecules. Additionally, it can undergo strain-promoted azide-alkyne cycloaddition reactions (SPAAC) with molecules bearing DBCO or BCN groups.