Since estrogen receptors are steroid hormone receptors that induce the production of growth factors, such as Epidermal Growth Factor (EGF), Insulin-like Growth Factor-1 (IGF) or Transforming growth factor alpha (TGFα), which stimulate tumor cell proliferation, competitive estrogen–estrogen receptor inhibitors have shown their utility to decrease tumor cell proliferation [24,25,26]. The gene discussed is ESR1; the disease is neoplasm.