Ubiquitin-binding protein. Specifically recognizes and binds 'Lys-63'-linked ubiquitin. 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. Component of the ARISC complex, at least composed of UIMC1/RAP80, ABRAXAS1, BRCC3/BRCC36, BABAM2 and BABAM1/NBA1. Component of the BRCA1-A complex, at least composed of the BRCA1, BARD1, UIMC1/RAP80, ABRAXAS1, BRCC3/BRCC36, BABAM2 and BABAM1/NBA1. In the BRCA1-A complex, interacts directly with ABRAXAS1. Interacts with UBE2I. Interacts with NR6A1. Interacts with ESR1 (By similarity). Interacts with TSP57 (By similarity). 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.