We organize the FragGT fragment database according to gene type to allow for trivial replacement of fragments with identical connectivity. For example, Figure shows a sample of possible replacements for a 2,4-substituted pyrimidine with gene type 14#14. Retrieved fragments for a given gene type are ranked according to a fragment scoring method such as similarity to a seed fragment. This retrieval process can be invoked exhaustively for bioisosteric replacement or scaffold hopping, or sampling schemes can be added for incorporation into a de novo search algorithm.