In addition, ERα is routinely used as a diagnosis marker supporting the molecular classification of breast cancers [13–15] and remains an essential therapeutic target for hormone-dependent breast cancers, in particular through administration of tamoxifen (TAM) and/or aromatase inhibitors (AI), that both are very efficient in reducing the risk of cancer recurrence [16–18]. The gene discussed is CYP19A1; the disease is breast cancer.