However, in the prospective data from the Nurses' Health Study [18], the adverse effect of nulliparity was confined to ER+PR+ breast cancer, but early age at menarche was associated with an increased risk of both ER+PR+ and ER- PR- breast cancer and the adverse effect of delayed childbearing was observed for ER- PR- but not ER+PR+ breast cancer. This evidence concerns the gene PGR and breast cancer.