The discovery of uromodulin mutations as a cause of FJHN and MCKD2 raises a new question; Why the mutant uromodulin causes uricemic underexcretion and hyperuricemia? Moreover, an old and still unsolved question is now highlighted; What is the physiological function of uromodulin? Recent experimental data on intracellular trafficking of uromodulin mutants and histopathological findings on renal biopsy specimens of patients are introduced. Hypotheses on mechanisms of FJHN/MCKD2 and supporting and contrary experimental findings are reviewed.