N-(Azido-PEG3)-N-Boc-PEG4-acid is a PEG-based PROTAC linker with a terminal azide group and is used in the synthesis of PROTACsIn Vitro: PROTACs contain two different ligands connected by a linker; one is a ligand for an E3 ubiquitin ligase and the other is for the target protein. PROTACs exploit the intracellular ubiquitin-proteasome system to selectively degrade target proteins.