Investigators have reported that HER2-overexpressing cell lines possess higher basal levels of VEGF expression and that stimulation with the HER3 and HER4 ligand, heregulin β1, further enhances VEGF secretion [16], suggesting that upregulation of angiogenesis may contribute to the tumorigenicity of HER2-positive breast cancer. This evidence concerns the gene ERBB3 and breast cancer.