Even a simple classification of breast cancer as early-onset (occurring at pre-menopausal age) or late-onset (encountered at postmenopausal age) reveals that the former is generally an estrogen receptor-negative form that appears in higher-grade tumors, while the latter is typically estrogen receptor-positive and is typical for lower-grade forms of the disease [26–28]. The gene discussed is ESR1; the disease is breast carcinoma.