ERBB3 and breast cancer: In breast cancer cells,Rac1 is the downstream effector of the ERBB receptor, which mediates the migration reaction of ErbB1/EGFR ligands (such as EGF or transforming growth factor α) and ERBB3 ligands through GRB7-VAV2 [61].Rac1-GEF-Prex1 is an important mediator of the Rac1 reaction in breast cancer cells.