MDM2 and cancer: Also, our results showed that MDM2 and p53 could regulate each other and showed a negative correlation in the presence of CHX and MG132 (Fig. 4b), and to prevent protein degradation, the ubiquitination assay of ex vitro by transfection of cancer cells with FLAG-tagged ubiquitin showed that p53 ubiquitination was decreased by co-treatment with CTD and MG132 (Fig. 4c).