E3 ubiquitin-protein ligase involved in DNA damage response by promoting DNA resection and homologous recombination (PubMed:26502055, PubMed:26502057). Recruited to sites of double-strand breaks following DNA damage and specifically promotes double-strand break repair via homologous recombination (PubMed:26502055, PubMed:26502057). Two different, non-exclusive, mechanisms have been proposed. According to a report, regulates the choice of double-strand break repair by favoring homologous recombination over non-homologous end joining (NHEJ): acts by mediating ubiquitination of XRCC5/Ku80, leading to remove the Ku complex from DNA breaks, thereby promoting homologous recombination (PubMed:26502055). According to another report, cooperates with UBE2Ds E2 ubiquitin ligases (UBE2D1, UBE2D2, UBE2D3 or UBE2D4) to promote homologous recombination by mediating ubiquitination of RBBP8/CtIP (PubMed:26502057). Together with NLK, involved in the ubiquitination and degradation of TCF/LEF (PubMed:16714285). Also exhibits auto-ubiquitination activity in combination with UBE2K (PubMed:16714285). May act as a negative regulator in the Wnt/beta-catenin-mediated signaling pathway (PubMed:16714285). {ECO:0000269|PubMed:16714285, ECO:0000269|PubMed:26502055, ECO:0000269|PubMed:26502057}. This is the function of Ensembl gene identifier ENSG00000134758 (RNF138, ring finger protein 138).