Random Research highlight: By screening all the ligand binding sites in the Protein Data Bank, we have found that while it is geometrically possible that a loop, formed from a protein-chain with residues ZYX would impersonate another chain-loop with residues XYZ by a simple twisting of either the loop or the bound ligand, it almost never happens. This fact is rather surprising, and implies a notable asymmetry, since (i) loops in the folded proteins sometimes can be flexible enough to be twisted, but (ii) ligands are almost always extremely mobile before binding to the protein, therefore they can turn around and bind to residue-sequence ZYX as well. J. Bioinform. Comput. Biol. 07, 931 (2009).