Assessing the difficulty of inhibiting a specific protein by a small molecule can be highly valuable in risk-assessment and prioritisation of a new target. In particular, when the disease linkage for a number of targets is broadly similar, being able to identify the most tractable can have a significant impact on informing target selection. With an increasing focus against new and novel protein classes, being able to assess the most likely targets to yield lead-like chemical start points can guide the selection and the lead-generation strategy implemented. This study exploits protein-ligand docking studies on published protein x-ray crystal structures to provide guidance on the feasibility of identifying small molecule inhibitors against a range of targets.Response to Reviewers: 1) I am not convinced that a biased fragment library (the bias is introduced via the filter criteria for logP, MW, and other parameters) should be able to judge the tractability of any target in general. What if potent fragment binders could not idenitified via docking just because they were removed from the initial fragment collection because of the filter criteria ? This concern is somehow reflected already in Fig 7A and 7B. A more general characterization of the fragment library (as expressed by the mean score of t he top 10% docking hits) is only hardly able to distinguish between druggable and "prodrug" binding sites (SP) or "prodrug" and undruggable sites (XP) . A more rigorous approach could for instance be based on a random selection of compounds rather than on a biased fragment library.The key criteria used to build this fragment library is chemical/structural diversity (there is a reference to the details behind how this library was built). This ensures that the library covers a broad range of scaffolds and interaction types. This allows the mapping of a target binding site by fragment screening (experimentally or computationally). The reason for focusing this fragment library around the stated criteria is that fragment hits with properties in these ranges are suggested as being good chemical start points for fragment-based lead generation projects where achieving an orally bioavailable drug is the aim (i.e. a molecule largely compliant with Lipinski-like criteria). See the report below (also reference 12) from Astex on 'Rule of Three' criteria. Our fragment library, however, has not been filtered as harshly as this library as we want to maximise the information from the screen (which is to some extent addressing the issue raised by the reviewer). The other key reason for using this library is that published fragment libraries (filtered similarly to the library I have used in this study) have been used to assess drugability using experimental screening approaches as shown in the reference 4 in the manuscript (below). Therefore there is validation for the use of libraries filtered by these criteria to assess drugability experimentally so I believe it is a suitable fragment set to assess targets computationally. Due to ...