National Comprehensive Cancer Network (NCCN) guidelines currently recommend that postmenopausal women with estrogen receptor- (ER-) positive DCIS consider using tamoxifen or an aromatase inhibitor to reduce the risk of a second breast cancer, while noting that the benefit of endocrine therapy for women with ER-negative DCIS is uncertain. This evidence concerns the gene ESR1 and ductal breast carcinoma in situ.