Porins form large aqueous channels, facilitating the diffusion of small hydrophilic molecules through the outer membrane in a non-specific manner. Important aspects of the channel activity are the conductance, the voltage dependence and the pH sensitivity. This family consists of the general diffusion porins originating mainly from the Alpha subdivision of Proteobacteria. Although members of this family share common structural and functional properties with the porins of General Bacterial Porin Family 1 (GBP-1) and the Rhodobacter porin family, they show a low sequence similarity to them. Prediction methods and low resolution experiments suggest that proteins of this family form ß-barrels, which is a common structural feature of general diffusion porins. In addition to that, porins of this family seem to require trimerization in order to function properly.