ESR1 and breast cancer: BC is a heterogeneous disease, demonstrating substantial intrinsic heterogeneity in terms of genetic, epigenetic, and phenotypic modifications and metabolism and is also affected by the estrogen receptor (ER) status, progesterone receptor (PR) status, and human epidermal growth factor receptor 2 (HER2) status (Baliu-Piqué et al., 2020).