Photoaffinity labeling is a process by which a macromolecule can be labeled at or near its binding or active site. The method can be applied to proteins, nucleic acids, and lipids—all macromolecules—by chemists who need to identify the parts of these molecules that are significant for particular biological functions. Otherwise, x-ray crystallography can frequently determine the complete structure of proteins and sometimes of nucleic acids, although the method does not necessarily highlight the reactive groups in a biomolecule. Nuclear magnetic resonance spectroscopy is increasingly useful in illuminating such structures.