ESR1 and cancer: In this context, breast tumors are classified into four major subtypes: (i) luminal A tumors expressing both estrogen and progesterone receptors and lacking HER-2 receptors (ER+/PR+/HER-2−), (ii) luminal B tumors that express estrogen receptors and HER-2 but lack progesterone receptors (ER+/PR−/HER-2+), (iii) HER-2 receptor-positive cancers lacking hormone receptors (ER−/PR−/HER-2+), and (iv) triple-negative breast cancer (TNBC) that lack both hormone and HER-2 receptors (ER−/PR−/HER-2−) [4,5,6].