BACKGROUND & AIMS: Wilson's disease (WD) is characterized by hepatic copper overload and caused by mutations in the gene encoding the copper-transporting P-type adenosine triphosphatase (ATPase) ATP7B. ATP7B interacts with COMMD1, a protein that is deleted in Bedlington terriers with hereditary copper toxicosis. Here we characterized the implications of the interaction between COMMD1 and ATP7B in relation to the pathogenesis of WD. METHODS: Glutathione-S-transferase pull-down experiments, co-immunoprecipitations, immunofluorescence microscopy, site-directed mutagenesis, and biosynthetic labeling experiments were performed to characterize the interaction between COMMD1 and ATP7B and the effects of WD causing mutations. RESULTS: COMMD1 specifically interacted with the amino-terminal region of ATP7B. This interaction was independent of intracellular copper levels and of the expression of the copper chaperone ATOX1. Four WD patient-derived mutations in this region of ATP7B significantly increased its binding to COMMD1. Two of these mutations also resulted in mislocalization and increased degradation rate of ATP7B. Although COMMD1 did not affect copper-induced trafficking of ATP7B, it markedly decreased the stability of newly synthesized ATP7B. CONCLUSIONS: Our data implicate COMMD1 in the pathogenesis of WD and indicate that COMMD1 exerts its regulatory role in copper homeostasis through the regulation of ATP7B stability.