[Isoform 1]: E3 ubiquitin-protein ligase (PubMed:22529100). May facilitate apoptosis by inhibiting APC/C-Cdh1-mediated poly- ubiquitination and subsequent proteasome-mediated degradation of the pro-apoptotic protein MOAP1 (PubMed:19100260, PubMed:22529100). Regulates the G1/S transition of the cell cycle and DNA damage-induced G2 arrest by stabilizing CDKN1A/p21 (PubMed:23213251). Positively regulates CDKN1A/p21 stability by competing with DTL for CDKN1A/p21 binding, therefore disrupting DCX(DTL) E3 ubiquitin ligase complex- mediated CDKN1A/p21 ubiquitination and degradation (PubMed:23213251). {ECO:0000269|PubMed:19100260, ECO:0000269|PubMed:22529100, ECO:0000269|PubMed:23213251}. [Isoform 2]: Regulates the G1/S transition of the cell cycle and DNA damage-induced G2 arrest by stabilizing CDKN1A/p21 (PubMed:23213251). Positively regulates CDKN1A/p21 stability by competing with DTL for CDKN1A/p21 binding, therefore disrupting DCX(DTL) E3 ubiquitin ligase complex-mediated CDKN1A/p21 ubiquitination and degradation (PubMed:23213251). Negatively regulates the canonical NF-kappa-B signaling pathway via stabilization of CACTIN in an ubiquitination-independent manner (PubMed:26363554). . This is the function of Ensembl gene identifier ENSG00000204599 (TRIM39, tripartite motif containing 39).