Some data suggest that menin has a dual role in breast cancer, with menin playing a tumor suppressive role in the non-malignant mammary epithelium [267,268,269] and a tumor promoting role in sporadic estrogen receptor (ER)-positive and ER-negative breast cancers [267,268,269,270,271,272,273,274,275,276]. This evidence concerns the gene ESR1 and breast cancer.