Animal models of human diseases are of vital importance, both for understanding basic disease mechanisms, and for developing potential treatments. Recent advances in molecular biology have helped identify the lesions underlying many genetic diseases. In conjunction with the development of techniques for manipulating the mouse embryo the possibility of mimicking these diseases in vivo has been raised. The example of cystic fibrosis illustrates this potential very well, demonstrating the usefulness of mouse models for gaining an improved understanding of the disease in humans and for the testing of new treatments.