ESR1 and ductal breast carcinoma in situ: Endocrine therapy includes selective estrogen receptor (ER) modulators such as tamoxifen, which was approved in the US in 1977 for ER+ breast cancer, as well as aromatase inhibitors like anastrozole, which was found in NSABP-35 to be associated with a decreased rate of breast cancer events after breast conservation surgery for DCIS among postmenopausal patients relative to tamoxifen12.