ERBB2 and breast adenocarcinoma: The aim of this study, therefore, was to investigate the ability of the endogenous Her-receptor activating ligands EGF (Her-1 ligand)[5] and heregulin- β1 (Her-3 and Her-4 ligand)[23,24] to influence the efficacy of trastuzumab in Her-2 positive breast adenocarcinoma in order to gain a clearer understanding of what may occur when the biological context is altered.