However, no changes in ventricular CSF osmolarity was observed in patients with subarachnoid hemorrhage and subsequent post‐hemorrhagic hydrocephalus in comparison to cisternal CSF obtained from patients undergoing preventive vascular clipping of an unruptured aneurysm.[52] Nor was any such osmolarity increase detected in CSF (ventricular or lumbar) obtained from patients with normal pressure hydrocephalus.[53] As AQP‐mediated osmotic water flow requires an osmotic gradient, we suspect a limited contribution of AQP1 to hydrocephalus formation in these pathologies. The gene discussed is AQP1; the disease is normal pressure hydrocephalus.