In concordance, the HER2/HER3 dimerisation inhibitor, pertuzumab, exhibited activity in breast cancer cells resistant to trastuzumab through potent inhibition of HER3 ligand-induced morphogenesis, lending support to the hypothesis that compensatory activation of this pathway is a key mediator of trastuzumab resistance [6, 27]. This evidence concerns the gene ERBB3 and breast cancer.