4-Bromobutylphosphonic acid is an alkyl chain-based PROTAC linker that can be used in the synthesis of PROTACs^[1]. In 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^[1].