Selection of target proteins for PROTAC can be based on the following considerations: 1. Whether the protein is specific or not, non-specific proteins can cause off-target toxicity; 2. Whether the protein belongs to the molecules that can be degraded by PROTAC, and it has been reported that the targets that can be used for PROTAC include, kinases (MEK, KRAS, CDK, and Bcr/Abl), transcription factors (e.g. p53, STAT, RAR, ER, and AR), epigenetic factors (e.g., HDAC and BET bromodomain), and E3 ligases themselves (e.g., MDM2), among others; 3. Whether or not the protein has been reported to have a ligand or crystalline form that can be commercially engineered.