The conformational flexibility of target proteins continues to be a major challenge in accurate modeling of protein-inhibitor interactions. A fundamental issue, yet to be clarified, is whether the observed conformational changes are controlled by the protein or induced by the inhibitor. Although the concept of induced fit has been widely adopted for describing the structural changes that accompany ligand binding, there is growing evidence in support of the dominance of proteins' intrinsic dynamics which has been evolutionarily optimized to accommodate its functional interactions. The wealth of structural data for target proteins in the presence of different ligands now permits us to make a critical assessment of the balance between these two effects in selecting the bound forms. We focused on three widely studied drug targets, HIV-1 reverse transcriptase, p38 MAP kinase, and cyclin-dependent kinase 2. A total of 292 structures determined for these enzymes in the presence of different inhibitors and unbound form permitted us to perform an extensive comparative analysis of the conformational space accessed upon ligand binding, and its relation to the intrinsic dynamics before ligand binding as predicted by elastic network model analysis. Our results show that the ligand selects the conformer that best matches its structural and dynamic properties among the conformers intrinsically accessible to the protein in the unliganded form. The results suggest that simple but robust rules encoded in the protein structure play a dominant role in predefining the mechanisms of ligand binding, which may be advantageously exploited in designing inhibitors.anisotropic network model ͉ conformational flexibility ͉ ensemble of structures ͉ pre-existing equilibrium ͉ principal component analysis T he dynamic nature of proteins plays a critical role in molecular recognition. Understanding the determinants of ligandrecognition and -binding dynamics is a major challenge with impact on drug discovery. Yet, progress in this field has been impeded by the complexity and specificity of interactions, the multiplicity of conformations accessible under equilibrium conditions, and insufficient data on the structure and energetics of protein-ligand interactions.Two different models have been proposed for explaining the conformational changes observed between the bound and unbound forms of proteins. The classical view, which dates back to the original work of Koshland in 1958, proposes an induced fit mechanism whereby ligand binding induces conformational changes on the target protein. Such an onset of conformational changes could be plausible on a local scale, i.e., slight rearrangements in side chain reorientations or even transitions between isomeric states could be triggered by the ligand. However, the more cooperative changes observed in other complexes, including concerted rearrangements of entire domains, have challenged this classical concept. The second, alternate view, pioneered by Monod, Wyman, and Changeux (MWC model), has gained broad ...