CYP19A1 and breast cancer: Based on clinical research, postsurgical tamoxifen treatment decreases breast cancer mortality by 34%.324 Other SERMs, including raloxifene and toremifene, have also been found to efficaciously block cell growth in patients with estrogen-responsive breast cancers.325–328 Aromatase is a key enzyme implicated in estrogen biosynthesis and converts androgens to estrogens.