This will be done by adding extra copies of the gene using a mouse leukaemia virus modified to ensure it does not trigger cancer.

BBC: 'Brain cell transplant' for Alzheimer's

By combining the gene test for BRCA1 and BRCA2 with this extra genetic information, women who have a high number of the newly identified markers could find they have a nearly 100% risk of getting breast cancer.

BBC: DNA test reveals 80 markers for inherited cancer risk