MDM2 and cancer: 42 E3 ligases, consisting of 2 co-opted E3 ligases, MDM2 and DCAF11, and 40 potentially novel E3 ligases, such as TRIM28 and Peptidylprolyl Isomerase Like 2 (PPIL2), were highly expressed in all surveyed tumor types at both transcriptomic and proteomic levels (Fig. 3a, b), and these E3 ligases may be utilized for PROTAC in a pan-cancer manner, pending a careful survey of their expression in normal tissues, which is the subject of the next section.