A target molecule can be synthesized if and only if a viable synthesis route of reactions from the starting materials to the target can be found. This depends on the availability of suitable starting materials, (often also called building block molecules), and the chemical tractability of the reactions in the route. Also, the viability of a synthesis depends on fulfilling different requirements dependent on the stage of a molecular discovery project. For example, in hit-to-lead discovery or early lead optimization, to achieve short design-make-test cycle times, complex and time-consuming multi-step syntheses that could take weeks or months are usually undesirable. In contrast, in late lead optimization for high-profile first-in-class targets, more involved syntheses might be acceptable. Similarly, low-yielding or expensive reactions are tolerable when it helps to reach a goal quickly in earlier discovery phases, whereas later in scale up for production this is not the case. Taking this a step further, in material science or agro-chemistry, larger amounts of the compounds are often required and material cost needs to be low, hence syntheses typically need to be simpler compared to drug discovery. Thus, synthesizability is not an inherent molecular property, as it can only be indirectly predicted from molecular structure in isolation.