AQP1 and obstructive hydrocephalus: In particular, AQP1 expression is strongly though not uniformly increased in some benign papilloma lesions with a preserved morphological structure, predicting CSF hypersecretion, while complete loss of the papillary architecture in CP carcinoma entails a lack of AQP1 expression, which is associated with normal ventricular size or with obstructive hydrocephalus.