Potato cathepsin D inhibitor (PDI) is a glycoprotein of 188 amino acids which can inhibit both the aspartic protease cathepsin D and the serine protease trypsin. Here we report the first X-ray structure of PDI at a resolution of 2.1 angstrom showing that PDI adopts a beta-trefoil fold, which is typical of the Kunitz-family protease inhibitors, with the inhibitory loops protruding from the core. Possible reactive-site loops including one involving a unique disulphide and another involving a protruding 3(10) helix are identified and docking studies indicate the mode of action of this unusual hi-functional inhibitor. (C) 2015 Elsevier Inc. All rights reserved.