Recent clinical studies revealed: (a) 66% of breast cancer expressed ER-α and 70% respond to hormonal therapy, while prolonged therapy leads to complete refractory cancer even though 30% of refractory tumors show ER-α expression [60], (b) around 85% prostate cancer patients show a good response towards hormonal therapy initially, eventually refractory hormonal cancer within 18–24 months [61,62]. The gene discussed is ESR1; the disease is breast carcinoma.