Ubiquitin-binding protein that 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).
