Azido-PEG4-Thiol is a PEG-based PROTAC linker used for synthesizing PROTAC molecules. As a click chemistry reagent, it possesses an azide group that can undergo copper-catalyzed azide-alkyne cycloaddition reactions (CuAAC) with molecules containing alkyne groups. Additionally, it can participate in strain-promoted azide-alkyne cycloaddition reactions (SPAAC) with molecules containing DBCO or BCN groups.