The p53 tumor suppressor is mutated or functionally inactivated in all cancers. Two key negative regulators of p53 are MDM2 and MDMX. Both of these proteins bind to p53 and inhibit its transcriptional activity. MDM2 also functions as an ubiquitin E3 ligase towards p53 targeting it for proteasome-mediated degradation. In this dissertation, we investigate the mechanisms of p53 regulation by focusing on the role of MDMX. We show that MDMX binding to MDM2 through the RING domain enhances the ability of MDM2 to ubiquitylate p53 and target it for degradation. Furthermore, we show that disrupting the MDM2:MDMX complex results in p53 activation, indicating that heterocomplex formation is essential for p53 suppression. We also explored endogenous binding partners of MDMX that may affect its regulation. We find that the small acidic 14-3-3 proteins bind to the C-terminus of MDMX. 14-3-3 binding is phosphorylation-dependent, and we show that the pro-survival kinase Akt phosphorylates MDMX at serine 367. Phosphorylation of this residue leads to 14-3-3 binding and results in stabilization of MDMX at the protein level. Because MDMX stabilization results in mutual stabilization of MDM2 mediated through their RING:RING interaction, p53 activity is inhibited by the accumulating MDM2:MDMX complex. Phosphorylation modifications are frequently counteracted by dephosphorylation, therefore we also explored the role of protein phosphatase 2A (PP2A) in MDMX regulation. We find that three regulatory B subunits of PP2A interact with MDMX, although the consequences of this interaction are not fully understood. Future studies will reveal if dephosphorylation regulates MDMX. Taken together, our results give a clearer picture of the critical role of MDMX in p53 regulation.