All **data** were taken from the Protein Data Bank [29,30]. The analysis was limited to protein X-ray crystal structures. Multi-model refinements and structures containing only Cα atoms were discharged. Only structures refined at a resolution better than 2.5 Å were retained, and this resulted in a list of about 121,000 entries of the Protein Data Bank. These were randomly divided into 14 subsets, each containing 7000 entries. Each entry was contained in one subset only (no overlap). All the analyses were then performed on each of the 14 subsets.