CYP19A1 and breast carcinoma: Women with a prior history of breast cancer are typically excluded from such breast cancer prevention trials because they are fundamentally different from women without a history of breast cancer in several ways: 1) the risk of developing a secondary breast cancer is different from a primary; 2) treatments for breast cancer (such as a unilateral mastectomy, use of aromatase inhibitors) modify risk of a secondary breast cancer; and 3) they have clinically significant competing risk for mortality or recurrence of their initial cancer.