E3 ubiquitin-protein ligase. The BRCA1-BARD1 heterodimer specifically mediates the formation of 'Lys-6'-linked polyubiquitin chains and coordinates a diverse range of cellular pathways such as DNA damage repair, ubiquitination and transcriptional regulation to maintain genomic stability. Plays a central role in the control of the cell cycle in response to DNA damage. Acts by mediating ubiquitin E3 ligase activity that is required for its tumor suppressor function. Also forms a heterodimer with CSTF1/CSTF-50 to modulate mRNA processing and RNAP II stability by inhibiting pre-mRNA 3' cleavage. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Homo- and heterodimer. Heterodimer (RING-type zinc finger) with BRCA1. Heterodimer (via ANK repeats and BRCT domains) with CSTF1/CSTF-50. Component of the BRCA1-A complex, at least composed of the BRCA1, BARD1, UIMC1/RAP80, ABRAXAS1, BRCC3/BRCC36, BABAM2 and BABAM1/NBA1. Interacts with UBXN1. During S phase of the cell cycle, colocalizes with BRCA1 into discrete subnuclear foci. Can translocate to the cytoplasm. Localizes at sites of DNA damage at double-strand breaks (DSBs); recruitment to DNA damage sites is mediated by the BRCA1-A complex. Processed during apoptosis. The homodimer is more susceptible to proteolytic cleavage than the BARD1/BRCA1 heterodimer. It is uncertain whether Met-1 or Met-26 is the initiator.