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