The present invention relates to transgenic non-human animals, tissues or cells derivedtherefrom and methods of producing them. The transgenic non-human animals ortissues or cells derived therefrom provide a system capable of expressing humanproteins responsible for drug metabolism in place of the homologous endogenous non-humananimal proteins and for the controlled expression of human genes introduced intothe animal so that the expression of the human genes is regulated in a manner moreclosely analogous to that seen in humans.
