Aquaglyceroporin Aqp10, a protein channel for water and glycerol, selectively permeates urea and boric acid due to its unique structural features—report researchers from Japan. By comparing and modeling molecular pore structures in fish species, the team discovered that bulky amino acid residues reduce the pore size of Aqp10—blocking the transport of certain molecules. This not only explains the mechanism of selective permeability but also provides a framework for predicting functions of uncharacterized aquaglyceroporins.