The expression of estrogen receptor (ER) and progesterone receptor (PR)—that are collectively named hormone receptors (HR)—human epidermal growth factor receptor 2 (HER2) and of the proliferation marker, Ki67, defines BC subtypes which are categorized into: Lumina A (HR+, HER2-), Luminal B (HR+, HER2+ or HR+, HER2-; Ki67 high), HER2-enriched (HR-, HER2+), and Triple-Negative (TN: HR-, HER2-). The gene discussed is ESR1; the disease is breast cancer.