While it is reported that about 60% of human breast cancers are positive for GPR30 expression [68], that mammary tumors of BALB–neuT mice express high levels of this alternative ER has not been reported before to our knowledge and highlights that these transgenic mice represent a valuable model to investigate the crosstalk between GPR30 and ErbB2/neu in vivo [72]. Here, GPER1 is linked to breast cancer.