ESR1 and breast cancer: For example, compared to no endocrine therapy, 10-year use of aromatase inhibitors (with ovarian function suppression for premenopausal survivors) reduced the number of contralateral breast cancers from 11.2 to 6.0 per 100 women with primary ER+/HER2 cancers (5.2 contralateral cancers avoided), while 10 years of tamoxifen for reduced the number to 6.5 (4.7 contralateral cancers avoided).