The PROTAC molecule itself contains three distinct portions: a ligand for binding to the target protein, a ligand for binding to an E3 ligase, and a linker joining these two ligands.

PROTACs (proteolysis-targeting chimaeras) are bifunctional molecules that bring a target protein into spatial proximity with an E3 ubiquitin ligase to trigger target ubiquitination and subsequent proteasomal degradation. Effective redirection of ligase poly-ubiquitination activity toward a new substrate protein requires formation of a ligase:PROTAC:target ternary complex, an intermediate species that is crucial to the cellular activity of degrader molecules. A characteristic feature of PROTACs mode of action is their sub-stoichiometric catalytic activity that alleviates the requirement for target engagement and occupancy of traditional inhibitors. New PROTAC molecules designed guided by the crystal structure show exquisite selectivity for inducing cellular depletion of Brd4 over its BET family members Brd2 and Brd3.