ESR1 and breast cancer: To investigate the regulatory effects of β2M overexpression between ER+ and ER− breast cancer with HER2−, we transfected cells with pEGFP-C1-β2M (β2M overexpression group) or pEGFP-C1 (control) for 36 h, and then analyzed the possible relevant signaling molecules described above by real-time PCR and western blotting.