Given a target molecule, synthesizability scoring approaches provide numerical or categorical proxy scores for the ease of synthesis, without performing time-consuming complete synthesis planning. The advantage of such scoring functions is that they are very fast to compute, taking at most milliseconds per molecule, as they are often only based on molecular features of the target molecules. This means that they can be used as part of the scoring function in large scale VS and in the DND loop. Several different approaches to scoring have been proposed. First, scores can be based only on topological features of the molecular graph, and fragment analyses, resting on the assumption that molecules containing common fragments are easier to make, or involve a simplified or truncated, fast retrosynthetic analysis. Boda et al. and Huang et al. suggested scores combining of structural and reaction features, and simplified retrosynthesis planning. A widely used approach is SAScore, as proposed by Ertl and Schuffenhauer. It penalizes molecules with rare fragments mined from PubChem, and molecules with too many stereo-centers, certain ring-features, and spiro-centers . A disadvantage of such topological approaches is that they capture molecular complexity rather than direct difficulty of synthesis.