X-ray crystallography uses the diffraction of X-rays to determine the structure of macromolecules. Over 88% of the structures in the PDB derive from this experimental method. Many macromolecules and macromolecular complexes form crystals under the right conditions. These are routinely frozen to cryogenic temperatures and exposed to synchrotron X-ray radiation to give a diffraction pattern (see Figure 21). Once electron density is calculated for a crystal then a structural model can be fitted into it. Both the individual diffraction points and the structural model are archived in the PDB.