Scientists are closer to understanding the role of a gene that is closely linked to breast and ovarian cancer.
BRCA1 attaches to a protein called ubiquitin in a process vital to a specific type of "error free" DNA repair, a study has found.
Cells lacking this property of the gene, known as "ubiquitin ligase activity", become sensitive to agents that can damage DNA and increase the risk of cancer.
Lead researcher Dr Jo Morris, from the University of Birmingham, said: "We know that loss of BRCA1 is associated with a high risk of breast cancer, so getting to grips with understanding this gene has been a major aim of breast cancer research.
"This study may explain why some cancer predisposing mutations are found in the front part of the BRCA1 gene - the part that allows it to function as a ubiquitin ligase."
The team also found that BRCA1 relies on a partner protein, BARD1, to perform its ubiquitin attachment role.
The research is reported in the journal Nature Structural and Molecular Biology.
Ubiquitin is a small protein found in almost all human cells which helps to regulate other proteins in the body.
As well as DNA repair, it can influence cell division and apoptosis, the programmed cell death process that helps defend the body against harmful cells that are damaged or infected.