The marked loss of bone mass during adjuvant endocrine therapy (OFS + aromatase inhibitors, but also tamoxifen, which does not protect bone in premenopausal women and acts as a partial antagonist inducing bone loss) in premenopausal patients leads to high annual rates of BMD reduction, with a substantial risk of osteoporosis and fractures. The gene discussed is CYP19A1; the disease is osteoporosis.