BARD1 is an E3 ubiquitin-protein ligase. In complex with BRCA1 it forms a heterodimer that specifically mediates the formation of 'Lys-6'-linked polyubiquitin chains. This complex coordinates a diverse range of cellular pathways, including DNA damage repair, ubiquitination and transcriptional regulation, to maintain genomic stability. BARD1 also plays a central role in controlling the cell cycle in response to DNA damage, and its ubiquitin E3 ligase activity is required for its tumour suppressor function.

Beyond its partnership with BRCA1, BARD1 forms a heterodimer with CSTF1 (CSTF-50). This interaction modulates mRNA processing and RNA polymerase II stability by inhibiting pre-mRNA 3' cleavage. The BARD1 gene encodes a nuclear protein that binds the N-terminal region of BRCA1 both in vivo and in vitro. It shares homology with the two most conserved regions of BRCA1: the N-terminal RING motif and the C-terminal BRCT domain. The RING motif is a cysteine-rich sequence found in a variety of proteins that regulate cell growth, including products of tumour suppressor genes and dominant proto-oncogenes. BARD1 also contains three tandem ankyrin repeats. HGNC places it among the RING finger proteins, ankyrin repeat domain containing proteins, the BRCA1 B complex and RING finger E3 ubiquitin protein ligases. Multiple alternatively spliced transcript variants encoding different isoforms have been reported.

The BARD1-BRCA1 interaction is disrupted by tumorigenic amino acid substitutions in BRCA1. This implies that formation of a stable complex between the two proteins may be an essential part of BRCA1 tumour suppression. BARD1 itself may be a target of oncogenic mutations in breast or ovarian cancer, which keeps it a focus of DNA repair and cancer biology research.