In models of disease involving blood–brain barrier disruption, including brain tumor [22], brain abscess [46], subarachnoid hemorrhage [47], and status epilepticus [48], Aqp4 null mice develop more severe brain edema than wild-type mice, suggesting that the mode of edema formation (vasogenic versus cytotoxic edema) is important to define the role of AQP4. The gene discussed is AQP4; the disease is subarachnoid hemorrhage.