Wilson disease (WD) is an autosomal recessive defect of copper transport characterized by massive accumulation of copper in the liver, which can lead to liver failure. Mutations in a copper transporting ATPase (WND or ATP7B) have been shown to cause the disease. The toxic milk mouse mutant (tx) accumulates copper in the liver in a manner similar to that observed in patients with WD. However, some physiological differences between tx mice and human WD patients have cast doubts on whether this mutant mouse is a valid model for WD. In this paper we report the isolation of cDNA clones encoding the murine homologue of WND. The predicted amino acid sequence is 1462 amino acids and contains the same functional domains identified in human and rat WND. As in the rat, the fourth metal binding domain is apparently non-functional. Similar levels of a 7.5 kb WND mRNA were detected in liver and kidney from normal and tx mice, indicating that transcription of this gene was unaffected in the mutant mice. The coding sequence of WND cDNA from the tx mouse liver identified a single nucleotide difference between the normal DL mouse and the tx which is predicted to change methionine 1356 in the eighth transmembrane domain to valine. This methionine is conserved in all copper ATPases including those from bacteria and yeast. The conclusion that this is the causative mutation is supported by the recent mapping of tx and WND to the same region of mouse chromosome 8. Thus the tx mouse is presented as a valid model for studies of the role of WND in copper transport and for investigation of different treatment strategies for WD.