Single-crystal X-ray diffraction provides a powerful technique for precisely determining the positions of atoms in a crystal (Refs. 1, 2, which are excellent texts on crystallography). Since the first crystal structure was worked out in 1913 by W. H. Bragg, tens of thousands of crystals have been analyzed using this experimental technique. Today crystal structure analysis by X-ray diffraction is usually routine for molecules containing less than 50 or so nonhydrogen atoms. The results of these structure determinations have provided answers to key questions in nearly every area of chemistry, biochemistry, and molecular biology.