MDM2 and neoplasm: In contrast, a study by Cummins et al. found that another target substrate of USP7 is an E3 ligase, MDM2, and that MDM2 also has a regulatory effect on P53, which ultimately leads to the degradation of 53 and acts as an oncogene [14].This suggests that USP7 can act as an oncogene or a tumor suppressor gene depending on the role played by its protein substrate and the amount of its own expression levels.