AQP1 and papilloma: Down-regulation of AQP1 has been reported in animal studies and in a case report on a 15-month-old girl with choroid plexus hyperplasia (45), while a small case series of choroid plexus tumors showed a heterogeneous pattern of AQP1 in papillomas and a weak expression in a single case of choroid plexus carcinoma (46).