the phenomenon of phase bias, it is possible for an incorrect initial assignment to propagate through successive refinements, so satisfactory conditions for a structure assignment are still a matter of debate. Indeed, some spectacular incorrect assignments have been reported, including a protein where the entire sequence was threaded backwards. Density modification (phase improvement) Solvent flattening Histogram matching Non-crystallographic symmetry averaging Partial structure Phase extension See also Coherent diffraction imaging Ptychography Phase retrieval External links An example of phase bias An appropriate use of 'molecular replacement' References Crystallography Inverse problems