Preclinical studies supported a key function for HER3 in promoting the growth of HER2-positive breast cancer cells [22], and these patients are now prime candidates for trastuzumab (Herceptin, a monoclonal antibody against HER2), perstuzumab (Perjeta, a HER2 and HER3 dimerization inhibitor), and lapatinib (Tykerb, a tyrosine kinase inhibitor), as well as for other compounds directed against HER2 and/or downstream kinases (Table 2). This evidence concerns the gene ERBB2 and breast cancer.