It has been reported that sustained activation with nuclear accumulation of activated ERK1/2 transmitting antiproliferative signals.47 Activation of the MAP kinase ERK1/2 by GPR30 was mediated via EGFR transactivation.25 The present study revealed that GPR30/EGFR/ERK1/2 signals mediated G-1-induced downregulation of cyclin B and growth arrest of ER− breast cancer cells. Here, MAPK3 is linked to breast carcinoma.