TNBC is further subclassified as basal-like breast cancer (BLBC, ER-/PR-/HER2-/CK5/6+ or epidermal growth factor receptor [EGFR] +) and quintuple-negative breast cancer (ER-/PR-/HER2-/CK5/6-/EGFR-) and accounts for 15–25% of all breast cancer cases. The gene discussed is ESR1; the disease is breast cancer.