ESR1 and cancer: Negatively regulated by ERβ1; highly expressed at the cancer stem-like stage; overexpressed in aromatase inhibitor resistant estrogen, receptor-positive (ER+) breast cancer; differentially regulated in HER2-positive breast cancer; downregulated in endometrioid carcinoma; upregulated in head and neck cancer cell lines; expression pattern can distinguish the gastric cancer grades and the control; Downregulated in chronic myelogenous leukemia stem cells