Aquaporin-1 (AQP1) forms a water-specific channel that provides the plasma membranes of red cells and kidney pro, Xenopus/Amphibian,imal tubules with high permeability to water, thereby permitting water to move in the direction of an osmotic gradient. AQP1 is responsible for the Colton blood group system. Appro, Xenopus/Amphibian,imately 92% of Caucasians are Co(A+B-) (Ala-46), appro, Xenopus/Amphibian,imately 8% are Co(A+B+), and only 0.2% are Co(A-B+) (Val-46). Co(A-B-), which is very rare, is due to a complete absence of AQP1. AQP1 is pharmacologically inhibited by submillimolar concentrations of mercury.