HRG and breast cancer: Silencing the expression of P-Rex1 from breast cancer cells largely impairs the elevation in Rac-GTP levels in response to growth factors such as EGF (ErbB1/EGFR ligand) and heregulin/neuregulin-1 (HRG, ErbB3/ErbB4 ligand), drastically affecting the motile capacity of these cells [15, 19].