I am a medical anthropologist breast cancer researcher and co-author of Dressed to Kill: The Link Between Breast Cancer and Bras.  Genetics constitute a small (less than 10%) contribution to the cause of breast cancer.  It would be far better for these women if they were advised to stop wearing tight bras for long hours each day, ending the constriction of their lymphatic (immune) system.   Bra- caused constriction of the breast lymphatics and consequent lymphstasis results in decreased clearance of chemotherapy drugs from the breasts, and since these drugs are known to themselves cause cancer this may be a mechanism for future breast cancer in women who have already had cancer treatment.  

Bras trap toxins within the breast (including chemo drugs and other carcinogens from our petrochemically polluted world), which is why bra-free women have the same low incidence of breast cancer as men.  The tighter and longer the bra is worn, the higher the incidence rises, to over 100 times higher for a 24/7 bra user compared to bra-free.  Numerous studies now confirm the bra-cancer link, although the lingerie industry and the cancer detection industry (mammograms and now genetic testing) are actively covering up this link.

Genetics does not guarantee a woman will get cancer, nor will wearing a tight bra.  But the chances are much worse for the bra user.  The best form of prevention for every woman is to stop constricting her breasts with bras.