Some studies in patients with idiopathic hypercalciuria suggest a specific defect in calcium reabsorption in the proximal renal tubule. One of the highly expressed claudins in the proximal tubule (PT) is CLDN2 and any defect in CLDN2 predisposes to calcium oxalate stone formation due to impaired reabsorption of calcium. The gene discussed is CLDN2; the disease is idiopathic hypercalciuria.