In order to exploit the tumour-specific nature of ERBB2 expression for genetic prodrug-activation therapy, we have generated recombinant adenoviral, retroviral and plasmid vectors containing an expression cassette consisting of the ERBB2 promoter and herpes simplex virus thymidine kinase coding sequence. In the case of the adenoviral vectors, the expression cassette was introduced into the E1 or E3 region of the genome. All of the vectors were capable of sensitizing ERBB2-positive cells to the action of ganciclovir. In contrast to the retroviral and plasmid vectors, however, transduction with the adenoviral vectors also resulted in sensitization of ERBB2-negative cells to ganciclovir, infection of cell lines with a beta-galactosidase expressing adenovirus showed that the sensitizing effect was not due to adenoviral infection per as in all but one of the cell lines tested. This study demonstrates that the ERBB2 promoter can be used to induce ERBB2-dependent sensitization to ganciclovir when in the context of retroviral and plasmid vectors. Observations made in this study do, however, suggest that adenoviral vectors may not be the ideal system to engineer conditional expression, and possible explanation for this phenomenon are discussed.