ESR1 and breast cancer: Numerous reports have demonstrated the molecular mechanisms of GPER1-mediated cell signaling in breast cancer cells (Lappano et al. 2014) and GPER1 activation by the selective ER modulator tamoxifen has been demonstrated (Vivacqua et al. 2012, Li et al. 2013, Vaziri-Gohar & Houston 2016, Rouhimoghadam et al. 2018), the mechanisms that regulate GPER1 expression are not well understood.