In this project the investigators will use a drug that blocks the effects of progesterone called ulipristal acetate (UA, EsmyaTM) that is currently licenced in the treatment of fibroids of the uterus. When used to treat fibroids UA is very well tolerated with no increase in side effects compared to a placebo tablet. 30 women at increased risk of BC will be recruited and have magnetic resonance imaging (MRI) scan and mammogram followed by a biopsy of one breast. After 3 months of UA treatment the MRI will be repeated along with a biopsy from the other breast. The effects of UA on many different cell types in the biopsies, including the stem cells and also the tissues like collagen that support them will be examined in detail. The effects of UA treatment on gene and protein expression in the breast tissue will also be examined. The goal is to identify which women will be sensitive or resistant to UA as a BC prevention treatment. In addition changes in the MRI scans with UA treatment will be examined using several different analysis techniques to try and identify who will likely benefit from UA treatment in the future without the need for biopsies.