The Long-Evans Cinnamon (LEC) rat shows similarity to Wilson disease in many clinical and biochemical features. We have cloned cDNAs for the rat gene (Atp7b) homologous to the human Wilson disease gene (ATP7B) and have used them to identify a partial deletion in the Atp7b gene in the LEC rat. The deletion removes at least 900 bp of the coding region at the 3' end, includes the crucial ATP binding domain and extends downstream of the gene. Our results provide convincing evidence for defining the LEC rat as an animal model for Wilson disease. This model will be important for studying liver pathophysiology, for developing therapy for Wilson disease and for studying the pathway of copper transport and its possible interaction with other heavy metals.