Of the eight case–control and cohort studies examining the association between mammographic density and breast cancer risk by tumor hormonal status, six [26-31] observed increased risk of both ER- positive (ER+) and ER-negative (ER-) tumors among those with the most dense breast tissue, and two [32,33] observed increased risks for ER+ tumors only. This evidence concerns the gene ESR1 and breast cancer.