To efficiently function as an E3 ubiquitin ligase, MDM2 has to form dimers or higher order oligomers. MDM2 can homodimerize (Cheng et al. 2011) or heterodimerize with MDM4 (MDMX) (Sharp et al. 1999, Huang et al. 2011, Pant et al. 2011). Dimerization involves the RING domain of MDM2 and/or MDM4. Heterodimers of MDM2 and MDM4 may be particularly important during embryonic development (Pant et al. 2011).