The molecular classification of breast cancer consists of the luminal A and B subtypes, which overlap with ER+ breast cancer; the HER2-enriched subtype, which overlaps with HER2+ breast cancer and can be TP53-mutated or wild type; and basal-like breast cancer, which overlaps with TNBC, and the claudin-low breast cancer subtype [2]. The gene discussed is ESR1; the disease is breast cancer.