To develop a PROTAC (Proteolysis Targeting Chimera) molecule, the ligand of the protein of interest moiety should have an affinity towards the protein lower than 1 µM.
This is because a higher affinity (lower Kd, typically under 1 µM) makes it more effective for the PROTAC to recruit the E3 ligase and facilitate the ubiquitination and subsequent degradation of the target protein. A lower binding affinity may not effectively promote the desired proteolytic action.