Structure-based virtual screening utilizes the 3D structure of the target when searching for new hits. During the screening, predicted 3D structures of small molecules are fitted into the binding site of the experimentally determined or modeled 3D structure of the target (docking calculation). The 3D structures of thousands of large macromolecules have been already determined by X-ray crystallography or NMR spectroscopy and can be easily selected or uploaded in Mcule. Small molecules predicted to form critical interactions with the target get better (more negative) docking scores and are ranked higher.