ESR1 and breast cancer: BC is a heterogeneous disease with 5 subtypes identified due to Ki67 index, estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2/erbB2) expression: 2 ER-positive luminal subtypes (luminal A: low Ki67 level, and luminal B: high Ki67 level) and 3 ER-negative subtypes (HER2-enriched, basal-like/triple-negative and normal-like) [1, 3].