Aquaporin 4 is a type III water channel regulator with limited surface exposed residues. AQP4 has been cloned from mice, rats, and humans. Algorithms predicted a type III transmembrane protein with intracellular n- and c-termini (left panel shows topology). Resides predicted in the extracellular loops are shown in one letter code. Residues based on the crystal structure  that are predicted to be in the exposed extracellular loops are highlighted in either blue (Loop A) green (Loop C), or orange-red (Loop E). Differences between rat (same as mouse) and human sequences in the exposed loops are noted by single letter code in white.