Thiol-C9-PEG4 is a PEG-based linker for PROTACs which joins two essential ligands, crucial for forming PROTAC molecules. This linker enables selective protein degradation by leveraging the ubiquitin-proteasome system within cells.

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[1].