ER signalling plays a key role in BC development, in fact, up to 75% of all breast cancers are ERα positive (Allred et al. 2004; Osborne and Shiff 2011; Dai et al. 2016); therefore, the deregulation of downstream target proteins of ER could partially explain the underlying mechanisms of initiation and development of most of BC. The gene discussed is ESR1; the disease is breast cancer.