Summary of published PROTAC targets, ligands, and E3 ubiquitin ligases. PROTACs are heterobifunctional molecules that recruit specific protein targets to an E3 ubiquitin ligase resulting in the ubiquitination and degradation of the target. This figure summarizes the different E3 ubiquitin ligases, ligands, and protein targets that have been studied as part of PROTAC chemistry development. IAP, inhibitors of apoptosis protein; MDM2, mouse double minute 2.