Although the precise causes of breast cancer are unclear, we know what the main risk factors are. Still, most women considered at high risk of breast cancer do not get it, while many who do have no known risk factors. Among the most significant factors are advancing age and a family history of breast cancer. Risk increases slightly for a woman who has had a benign breast lump and increases significantly for a woman who has previously had cancer of the breast or the ovaries. 

A woman whose mother, sister, or daughter has had breast cancer is two to three times more likely to develop the disease, particularly if more than one first-degree relative has been affected. Researchers have now identified two genes responsible for some instances of familial breast cancer, called BRCA1 and BRCA2 Having the gene predisposes a woman to breast cancer but does not ensure that she will get it. These genes cause about one in 20 breast cancers.