PGR and cancer: Here we sought to investigate the merit of targeting progesterone signalling as a means of cancer prevention by (i) comparing the levels of progesterone during one menstrual cycle in women with and without a BRCA mutation, (ii) developing markers which are indicative of a progesterone-mediated field cancerization in the breast and (iii) examining the ability of progesterone receptor antagonists (mifepristone and ulipristal acetate) to reduce the level of field cancerization in normal breast tissue.