ESR1 and breast cancer: Although the expression of the estrogen receptor (ER), the progesterone receptor (PR) and the ERBB2 receptor (HER2) has laid the foundation for the classification of breast cancer, breast cancer is divided into at least five molecular subtypes (i.e., Luminal A, Luminal B, Her2-enriched, Basal-like and Normal-like) based on gene expression.