Scientists have discovered that women who have a strong history of breast cancer in their family are more at risk of developing the disease, even if they do not have a high-risk gene.
Women who do not have a faulty BRCA gene but have close relatives with breast cancer are four times more likely than average to get breast cancer, according to research.
However, if women do have a defective BRCA1 or BRCA2 gene, they have an 80% chance of developing breast cancer.
The study by a team of Canadian-led scientists looked at a group of women with one first-degree relative under 50 with breast cancer and also at least one other relative with the disease, or three relatives of any age.
Although none of them had a faulty BRCA gene, the proportion of women who developed breast cancer was one in three compared with an average rate for the general population of one in nine.
Study leader, Dr Steven Narod, from the University of Toronto, said: "This is the first time the breast cancer risk for this group of women has been measured, and it's significantly higher than that of the general population."