There are over 600 E3 ligases in humans, and only a few of them are currently capable of being used in PROTAC technology, such as VHL, CRBN, Mdm2, and cIAP. However, these E3 ligases lack tissue specificity that is impactive for target specificity in the PROTAC technology. In addition, most of the ligands binding to those E3 ligases have high molecular weights, which challenge the oral drug development via PROTAC technology. In order to overcome the major challenges faced by the entire pharmaceutical industry, Aubrak has discovered a new type of E3 ligase that is specifically expressed in tumor tissues, and also discovered its specific ligands. These new discoveries provided our PROTAC system with better target specificity and can be applied to various PROTAC drug discovery and development projects.