Role of microRNA in endocrine resistance. microRNAs (miRNAs) that regulate the growth, survival, apoptosis, epithelial-to-mesenchymal transition (EMT), and metastasis of breast cancer cells are implicated in the loss of responsiveness to endocrine therapies. miRNAs that are upregulated in endocrine resistance (red) could potentially be targets of RNA interference therapies, while miRNAs that are downregulated in endocrine resistance (green) could be targets of a replacement therapy in endocrine-resistant breast tumors. (A) miRNAs involved in tamoxifen resistance. (B) miRNAs involved in aromatase inhibitor (AI) resistance. Bim, Bcl-2-like 11; EGFR, epidermal growth factor receptor; ER, estrogen receptor; E2, 17β-estradiol; FGFR1, fibroblast growth factor receptor 1; HER2, human epidermal growth factor receptor 2; IGFR1, insulin-like growth factor receptor 1; MAPK, mitogen-activated protein kinase; MTDH, metadherin; PI3K, phosphoinositide 3-kinase; PTEN, phosphatase and tensin homolog; TGFR1β, transforming growth factor beta receptor 1; ZEB1/2, zinc finger E box-binding homeobox 1/2.