This paper reports a computational method, the quantized elastic deformational model, that can reliably describe the conformational flexibility of a protein in the absence of the amino acid sequence and atomic coordinates. The essence of this method lies in the fact that, in modeling the functionally important conformational changes such as domain movements, it is possible to abandon the traditional concepts of protein structure (bonds, angles, dihedrals, etc.) and treat the protein as an elastic object. The shape and mass distribution of the object are described by the electron density maps, at various resolutions, from methods such as x-ray diffraction or cryo-electron microscopy. The amplitudes and directionality of the elastic deformational modes of a protein, whose patterns match the biologically relevant conformational changes, can then be derived solely based on the electron density map. The method yields an accurate description of protein dynamics over a wide range of resolutions even as low as 15-20 Å at which there is nearly no visually distinguishable internal structures. Therefore, this method dramatically enhances the capability of studying protein motions in structural biology. It is also expected to have ample applications in related fields such as bioinformatics, structural genomics, and proteomics, in which one's ability to extract functional information from the not-so-well-defined structural models is vitally important.conformational flexibility ͉ elastic deformation ͉ large conformational change ͉ elastic network ͉ normal mode analysis C omputational simulations of protein dynamics (1, 2) play an important role in deciphering protein functions in modern structural biology. To date, all the procedures for describing the motions of a protein require the knowledge of the atomic coordinates-i.e., the precise locations of the atoms. However, as the field of structural biology moves into an era of supermolecular complexes and membrane-bound proteins, there have been an increasing number of cases in which one can only obtain fuzzy images of the molecules by means of, for example, cryoelectron microscopy (cryo-EM). The knowledge of structures in these cases is not much more than the rough shapes of the molecules. Therefore, a challenging question is whether one can describe the motions of a protein, at least the gross features, based on its fuzzy image. The success of such a method would not only advance one's ability to model protein motions to a completely new level in structure biology, but it will also profoundly inf luence broader fields such as bioinformatics, structural genomics, and proteomics, in which one's ability to extract functional information from the not-so-well-defined structural models is vitally important.In the light of such a challenge, we developed a computational method, the quantized elastic deformational model (QEDM), by combining and extending several existing methods that were developed for related, but different, purposes. The results clearly demonstrate that, without the...