ERBB4 and cancer: Unlike many other cancers, PCa tumours express high EGFR and ErbB3, low HER2, and mostly lack ErbB4 expression.11,12 Single inhibitors of EGFR and HER2 (e.g. gefitinib, erlotinib and trastuzumab) that were successful in other cancers, failed in PCa clinical trials13–17 and our data implicated ErbB3 signalling in this resistance.18 Lapatinib (the first, FDA-approved, small-molecular dual-HER2/EGFR tyrosine kinase inhibitor (TKI) for treatment of HER2+/ErbB2+ breast cancers19) was ineffective in PCa clinical trials.