Ubiquitin-binding protein. Specifically recognizes and binds 'Lys-63'-linked ubiquitin (PubMed:19536136). Plays a central role in the BRCA1-A complex by specifically binding 'Lys-63'-linked ubiquitinated histones H2A and H2AX at DNA lesions sites, leading to target the BRCA1-BARD1 heterodimer to sites of DNA damage at double-strand breaks (DSBs). The BRCA1-A complex also possesses deubiquitinase activity that specifically removes 'Lys-63'-linked ubiquitin on histones H2A and H2AX. Also weakly binds monoubiquitin but with much less affinity than 'Lys-63'-linked ubiquitin. May interact with monoubiquitinated histones H2A and H2B; the relevance of such results is however unclear in vivo. Does not bind Lys-48'-linked ubiquitin. May indirectly act as a transcriptional repressor by inhibiting the interaction of NR6A1 with the corepressor NCOR1 (By similarity). Component of the ARISC complex, at least composed of UIMC1/RAP80, ABRAXAS1, BRCC3/BRCC36, BABAM2 and BABAM1/NBA1 (By similarity). Component of the BRCA1-A complex, at least composed of the BRCA1, BARD1, UIMC1/RAP80, ABRAXAS1, BRCC3/BRCC36, BABAM2 and BABAM1/NBA1 (By similarity). In the BRCA1-A complex, interacts directly with ABRAXAS1 (By similarity). Interacts with ESR1 (By similarity). Interacts with UBE2I (By similarity). Interacts with NR6A1 (PubMed:7760852). Interacts with TSP57 (PubMed:12954732). Interacts with TRAIP (By similarity). Localizes at sites of DNA damage at double-strand breaks (DSBs). The tandem UIM domains form a continuous 60 Angstrom-long alpha-helix and mediate binding to 'Lys-63'-linked ubiquitins. UIM1 and UIM2 bind to the proximal and distal ubiquitin moieties and recognize an 'Ile-44'-centered hydrophobic patch. Since UIMs don't interact with the 'Lys-63' isopeptide bond the UIM-linker region between the 2 UIM domains determines the selectivity for 'Lys-63'-linkage, and its length is very important for specificity. The Abraxas-interacting region (AIR) mediates the interaction with ABRAXAS1. Sumoylated. Phosphorylated upon DNA damage by ATM or ATR. Belongs to the RAP80 family. Extended N-terminus.