p63.

The p63 gene, a member of the p53 gene family, is expressed into at least six protein isoforms which are divided into two groups, those containing the transcription activation domain (TA isoforms) and those that do not (Delta N isoforms). The TA isoforms are similar to p53 in that they are able to activate transcription of specific target genes and induce cell cycle arrest and apoptosis. The Delta N isoforms are unable to activate transcription, and act in a dominant negative manner, inhibiting transcription activation by both p53 and TA isoforms.

Mdmx stabilizes p53 and Mdm2 via two distinct mechanisms.

The p53 protein maintains genomic integrity through its ability to induce cell cycle arrest or apoptosis in response to various forms of stress. Substantial regulation of p53 activity occurs at the level of protein stability, largely determined by the activity of the Mdm2 protein. Mdm2 targets both p53 and itself for ubiquitylation and subsequent proteasomal degradation by acting as an ubiquitin ligase, a function that needs an intact Mdm2 RING finger.