The application of abiraterone acetate/prednisone combined with exemestane in ERα-positive breast cancer patients (NCT01381874) previously treated with nonsteroidal aromatase inhibitors resulted in no improvement in PFS compared to exemestane, which led to the speculation that the lack of clinical benefit was due to abiraterone acetate increasing levels of progesterone [105]. This evidence concerns the gene CYP19A1 and breast cancer.