Breast cancer is heterogeneous with distinct molecular and histological features that can be ascribed into luminal A-like (ER+, PR+, HER2-), luminal B-like HER2-, luminal B-like HER2+, HER2-enriched (non-luminal) and triple-negative (ER-, PR-, HER2-) in current clinical practice (175). The gene discussed is ESR1; the disease is breast cancer.