Lactic acidosis, which takes place in the brain during cerebral ischemia [37], seems to play a role in the development of cytotoxic edema [40], since lactic acid leads to swelling in cultured rat astrocytes; moreover, an increase of AQP4 expression has been demonstrated on the astrocyte cell membrane as well as AQP9 permeability [27]. The gene discussed is AQP4; the disease is brain ischemia.