Water is a critical component of all living cells. Interestingly, tissue membranes show a great degree of water permeability. Mammalian red cells, renal proximal tubules, and descending thin limb of Henle are extraordinarily permeable to water. Water crosses hydrophobic plasma membranes either by simple diffusion or through a facilitative transport mechanism mediated by special protein aquaporins. Aquaporin 0 or MIP26 (major intrinsic protein 26kD), and Aquaporin 1 has been the foundation of the growing family of aquaporins. The lens specific Aquaporin 0 represents up to 80% of total lens membrane protein. Defects in Aquaporin 0 are a cause of autosomal recessive congenital cataract. The lens opacity mutation (LOP) is an AA substitution that inhibits targeting of MIP to the cell membrane. Human Aquaporin 0 is a 263 amino acid transmembrane protein belonging to the MIP family. Aquaporin families of proteins are predicted to contain six transmembrane domains. The N and C terminus are predicted to be cytoplasmic.