Tumor suppressor that is required to sustain G2/M checkpoint after DNA damage (PubMed:20843368, PubMed:20154723, PubMed:24240685). Acts as a p53/TP53 activator by inhibiting MDM2 binding to p53/TP53 and stimulating non-proteolytic polyubiquitination of p53/TP53. Exhibits ubiquitin ligase (E3) activity and assemble ubiquitin polymers through 'Lys-11'- (K11-), 'Lys-29'- (K29-) and 'Lys-63'- (K63)-linkages, independently of the ubiquitin-conjugating enzyme (E2). Promotes p53/TP53-dependent transcription of CDKN1A/p21, leading to robust checkpoint response (PubMed:24240685). Mediates CDKN1A/p21 protein stability in a ubiquitin-independent manner. Interacts with HDAC1 and prevents binding of HDAC1 to CDKN1A/p21 and facilitates the acetylation and stabilization of CDKN1A/p21 (PubMed:20154723). May have a role in the assembly of primary cilia (By similarity). Interacts with HDAC1; the interaction prevents binding of HDAC1 to CDKN1A/p21 and facilitates the acetylation and stabilization of CDKN1A/p21 (PubMed:20154723). Interacts with p53/TP53; the interaction inhibits binding of p53/TP53 and MDM2 (PubMed:24240685). Localizes to the actin cytoskeleton in a proportion of cells. Colocalizes with centriolar acetylated tubulin (By similarity). Highly expressed in testis. Weak expression found in brain, lung, heart, ovary, thymus, spleen and kidney. Upon DNA damage by agents such as ionizing radiation, UV and actinomycin D. Activated by p53/TP53.