The linker is responsible for linking two key domains of PROTAC: the domain that specifically binds to the target protein that needs to be degraded, and the domain that binds to the E3 ubiquitin ligase. To date, several types of connectors have been reported and used in the formation of PROTAC ternary complexes, such as PEG liner, alkyl liner and "click chemical" liner. It plays an important role in the effective ubiquitination and final degradation of target proteins.