It has been reported that women diagnosed with estrogen receptor-positive (ER+)/progesterone receptor-positive (PR+) tumors are more responsive to hormonal treatment and have a better prognosis than those diagnosed with estrogen receptor-negative (ER-)/progesterone receptor negative (PR-) tumors, suggesting etiologic heterogeneity of hormone-receptor defined subtypes of breast cancer [8], [9]. The gene discussed is ESR1; the disease is breast cancer.