PGR and breast cancer: Nevertheless, there are a few studies that have evaluated pre-menopausal cases – a cohort33 and a population-based case-control34 study which reported an increased risk of breast cancer with smoking in ER+ –, or, in post-menopausal women, two case-control studies7,35 which found no association between smoking and ER+ breast cancer, and a Norwegian nested case-control15, including women aged 50–69 years, that found an increase of risk for ER+ and ER-/PR+ tumors.