A mutation in or lack of genes, such as those for aquaporin 2 (AQP2), Barttin, sodium-potassium-chloride cotransporter 2 (NKCC2), or V2 vasopressin receptor, results in polyuria and hydronephrosis [37,38,39,40,41,42], and polyuria can overwhelm the urine excretion capacity to induce hydronephrosis. Here, AQP2 is linked to hydronephrosis.