ATP7B and Wilson disease: In the mouse model of Wilson disease, as in humans, Gray et al. demonstrated that large amounts of excess Cu accumulate when the Cu “pump” ATP7B is defective in hepatocytes.38 This was accompanied by a large increase in urinary Cu excretion (normally very low) in the form of a small molecule (less than 3 kDa).38 As shown below, blood plasma samples from two of the same Wilson disease model mice were then analyzed for total Cu as well as Cu in 3 kDa ultrafiltrates (Fig. 1A).