For example, E2 was shown to upregulate brain‐derived neurotrophic factor (BDNF) and epidermal growth factor in ER+ astrocytes, which then activate their cognate receptors tropomyosin receptor kinase B (TrkB) and EGFR in breast cancer cells, promoting tumor‐initiating capability, migration and invasion. This evidence concerns the gene BDNF and breast carcinoma.