Approximately 70% of breast cancers express ERα [1,2], and its presence in breast tumors is routinely used to predict a response to endocrine therapy such as tamoxifen - an anti-estrogen that blocks estrogen-stimulated breast cancer cell growth - or aromatase inhibitors (AIs) - agents that suppress estrogen synthesis in the body. This evidence concerns the gene CYP19A1 and breast carcinoma.