In this study, we focused on metalloproteins containing Mg 2+ , Mn 2+ , and Zn 2+ . To compile a data set that would later allow to extract metal-chelating groups contained in small molecules, the PDB was first searched for proteins containing the metal ions of interest. This resulted in over 16,000 protein structures containing either Mg 2+ or Zn 2+ (Table , complete list with PDB IDs in S1 of the Supporting Information). In contrast, Mn 2+ was present in only approximately 3,300 protein structures. Using the batch download script provided by the PDB, over 98% of the retrieved structures could be downloaded. Only 44% of the downloaded structures containing Mg 2+ and Mn 2+ and 20% containing Zn 2+ had a ligand within 2.6 Å of the metal ion.