Breast cancer (BC) is the commonest cancer, affecting 1.4 million women per year of whom a third die from the disease. There is an urgent need for new approaches to BC prevention. Progesterone is a hormone that is produced naturally from a woman's ovaries during each menstrual cycle and is often used in hormone replacement therapy (HRT) after the menopause. When used in HRT progesterone increases the risk of BC and BC death. In experiments in mice and rats progesterone has been shown to increase the growth of the normal mammary gland (the rodent breast). When human breast tissue is grown in the laboratory it also grows in response to progesterone. In particular progesterone has been shown to increase the growth of particular cells called stem cells which survive for a long time in the breast. It is thought that the exposure of these stem cells to progesterone over many years is the reason that women whose periods start early and finish late or those who do not have a pregnancy to interrupt their menstrual cycles and those that take HRT all have an increased risk of BC.