Shape is the critical concept. Like all proteins, ligands and receptors are made up of many different amino acids strung together like pearls and folded into distinct three-dimensional shapes. A ligand with the right shape fits its matching receptor like a key fits a lock. By using sophisticated molecular engineering techniques, the researchers can change the lineup of amino acids in a ligand, essentially making millions of keys that they then screen to see which might unlock its matching receptor in some desirable way. A key that fits better and trips the lock more efficiently -scientists call this a superagonist—might...