Additional research is needed to elucidate the mechanisms underlying the observed associations between breast density and risk of breast cancer.31,32 For example, a potential mechanism is hypercellularity and increased breast epithelium in dense breasts that may lead to increased rates of somatic mutations.33 In addition, dense breasts contain a large amount of stroma with aromatase activity, which has been associated with an increased release of estrogen that may lead to carcinogenesis.34 Further studies are needed to elucidate these mechanisms among older women. This evidence concerns the gene CYP19A1 and breast carcinoma.