Both isoforms seemed significantly more expressed in the less severe BC, i.e. ductal carcinoma in situ (DCIS) and luminal A/B (ER+ and/or PR+), and decreased with increasing BC severity, i.e. Her2+ (ER−/PR−/HER2+) and triple negative tumors (ER−/PR−/HER2−). Here, ESR1 is linked to breast cancer.