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Recent years have witnessed significant advancements in X-ray data-acquisition techniques and phasing algorithms, which have made possible the successful use of a very small anomalous diffraction signal for the solution of crystal structures of macromolecules. Two crystal structures, a 44 kDa glucose isomerase containing nine sulfurs and a 33 kDa xylanase containing five sulfurs, have been solved from single-wavelength anomalous data using widely available methods and programs. These two enzymes contain less sulfur than most proteins in the bacterial or eukaryotic proteomes, providing a Bijvoet ratio of about 0.6%. For glucose isomerase the automatically interpretable electron-density maps could be obtained at high as well as low resolution. The S-SAD approach relies on the anomalous signal of sulfur naturally occurring in proteins and alleviates all need for sample derivatization. It may therefore be …