Based on gene expression profiling breast cancer is categorized into five intrinsic subgroups: luminal A (mostly estrogen-receptor (ER)-positive and histologically low-grade), luminal B (also predominantly ER-positive and often high-grade), HER2-enriched (often show amplification and high expression of the ERBB2 gene), basal-like (mostly corresponding to ER-negative, progesterone-receptor (PR)–negative and HER2-negative tumors thus triple negative breast cancer (TNBC)) and normal-like cases, which clinical relevance is still controversial [3–6]. This evidence concerns the gene ESR1 and breast cancer.