ESR1 and breast cancer: Using HR (ERα and PR) and ERBB2 status, we also subdivided the total population (n = 456) into four breast cancer molecular subtypes: HR+/ERBB2+ (n = 54), HR+/ERBB2− (n = 289), HR−/ERBB2+ (n = 45) and HR−/ERBB2− (n = 68).