ESR1 and breast cancer: These biomarkers contributed to the classification of five major subtypes of breast cancer: Luminal A (i.e. ER positive (ER+)/PR positive (PR+), HER2 negative (HER2−); Luminal B (i.e. ER+/PR+, HER2+ or HER2− with high levels of Ki-67); Triple-negative/basal-like (i.e. ER−/PR−, HER2−); HER2-enriched (i.e. ER−/PR−, HER2+), and Normal-like (ER+/PR+, HER2− with low levels of Ki-67), being the triple-negative subtype predictive off the worst prognosis.