CYP19A1 and breast cancer: As the female sex hormone estrogen influences the occurrence of breast cancer (BC), a large proportion of the tumors (80%) have the potential for hormone-dependent growth.1 This fact is used in modern endocrine BC therapy by either blocking estrogen’s mechanisms of action in the tumor with tamoxifen or reducing the availability of body-specific estrogen in the tissue with aromatase inhibitors (AI) or by castration treatment with gonadotropin-releasing hormone (GnRH).