Hormonal risk factors associated with an increased risk of breast cancer and related to oestrogen exposure. For references supporting the validity of this figure, see Santen . E, oestrogen; E2, oestradiol; HRT, hormone replacement therapy; OOX, oophorectomy; P, progesterone. Reproduced with permission from Santen RJ: Endocrine-responsive cancer. In Williams Textbook of Endocrinology. Edited by Larsen PR, Kronenberg HM, Melmed S, Polonsky KS. Philadelphia, PA: WB Saunders Company; 2007:1763–1801. © Elsevier 2007.