Accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. Catalyzes monoubiquitination. Involved in mitomycin-C (MMC)-induced DNA repair. Acts as a specific E2 ubiquitin-conjugating enzyme for the Fanconi anemia complex by associating with E3 ubiquitin-protein ligase FANCL and catalyzing monoubiquitination of FANCD2, a key step in the DNA damage pathway (PubMed:16916645, PubMed:17938197, PubMed:19111657, PubMed:19589784, PubMed:28437106). Also mediates monoubiquitination of FANCL and FANCI (PubMed:16916645, PubMed:17938197, PubMed:19111657, PubMed:19589784). May contribute to ubiquitination and degradation of BRCA1 (PubMed:19887602). In vitro able to promote polyubiquitination using all 7 ubiquitin Lys residues, but may prefer 'Lys-11'-, 'Lys-27'-, 'Lys-48'- and 'Lys-63'-linked polyubiquitination (PubMed:20061386). S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Directly interacts with FANCL (PubMed:16916645, PubMed:17938197, PubMed:19111657, PubMed:21775430, PubMed:24389026). Interacts with BRCA1 (PubMed:19887602). Accumulates to chromatin. Down-regulated following hypoxia. Up-regulated in breast cancers. Auto-ubiquitinated. Effects of auto-monoubiquitination at Lys-91 and Lys-182 are unclear: according to a report, monoubiquitination inactivates E2 enzyme activity (PubMed:16916645). In contrast, according to another report, autoubiquitination does not affect E2 enzyme activity (PubMed:19111657). The disease is caused by variants affecting the gene represented in this entry. Belongs to the ubiquitin-conjugating enzyme family.