Predicted or estimated protein chemical shifts can be used to assist with the chemical shift assignment process. This is especially true if a similar (or identical) protein structure has been solved by X-ray crystallography.  In this case, the three-dimensional structure can be used to estimate what the NMR chemical shifts should be and thereby simplify the process of assigning the experimentally observed chemical shifts.  Predicted/estimated protein chemical shifts can also be used to identify incorrect or mis-assignments, to correct mis-referenced or incorrectly referenced chemical shifts, to optimize protein structures via chemical shift refinement and to identify the relative contributions of different electronic or geometric effects to nucleus-specific shifts. Protein chemical shifts can also be used to identify secondary structures, to estimate backbone torsion angles, to determine the location of aromatic rings, to assess cysteine oxidation states, to estimate solvent exposure and to measure backbone flexibility.