ESR1 and breast cancer: In a meta-analysis including 15 studies (3 prospective cohorts, 10 case-control studies and 2 pooled analyses), ever breastfeeding was associated with a reduced risk of developing both luminal A and TN subtypes12; while in a larger meta-analysis, including 27 distinct studies (8 prospective cohorts and 19 case– control studies) ever breastfeeding was inversely associated with ER- and PR- BC as well as TN BC, but no significant association was observed with ER+ PR+ BC overall23.