HIF-1α positivity was more common in contralateral breast cancer (CBC) during tamoxifen adjuvant therapy (N = 60) than in CBC without prior tamoxifen (N = 522) (32% (18/56) versus 17% (80/482) (64) These reports highlight the role of HIF-1α as a prognostic marker, but also demonstrate the positive association between HIF-1α overexpression and endocrine therapy resistance in breast cancer. Here, HIF1A is linked to breast cancer.