Along with MDM2, contributes to TP53 regulation (PubMed:32300648). Inhibits p53/TP53- and TP73/p73-mediated cell cycle arrest and apoptosis by binding its transcriptional activation domain. Inhibits degradation of MDM2. Can reverse MDM2-targeted degradation of TP53 while maintaining suppression of TP53 transactivation and apoptotic functions. Interacts with MDM2 (PubMed:16163388, PubMed:19838211, PubMed:32300648). Interacts with TP53, TP73 and USP2. Found in a trimeric complex with USP2, MDM2 and MDM4. Interacts (phosphorylated) with YWHAG; negatively regulates MDM4 activity toward TP53. Additional isoforms seem to exist. Expressed in all tissues tested with high levels in thymus. Down-regulated by cisplatin (at protein level). Region I is sufficient for binding TP53 and inhibiting its G1 arrest and apoptosis functions. It also binds TP73. Region II contains most of a central acidic region and a putative C4-type zinc finger. The RING finger domain which coordinates two molecules of zinc mediates the heterooligomerization with MDM2. Phosphorylated. Phosphorylation at Ser-367 promotes interaction with YWHAG and subsequent ubiquitination and degradation. Phosphorylation at Ser-342 also induces ubiquitination and degradation but to a lower extent. Ubiquitinated and degraded by MDM2. Deubiquitination by USP2 on the other hand stabilizes the MDM4 protein. The disease is caused by variants affecting the gene represented in this entry. Cancer-specific isoform, may counteract MDM2/MDM4-mediated p53 degradation. Belongs to the MDM2/MDM4 family.