It was reported that 7–16 mature oocytes were obtained under 500 pg/ml serum E2 concentrations on the day of induced ovulation, when COS with aromatase inhibitor was performed to women with breast cancer [17–19], and that number of mature oocytes and fertilization rates were not inferior to spontaneous COS [17, 19]. This evidence concerns the gene CYP19A1 and breast cancer.