We next describe the procedure required for every combination of a valid cutset F (q) and a specific synthon reaction R. The connected components in F (q) differ from the subsubstructures that we have to match in the synthon sets of the synthon reaction only in that they contain unlabeled connector vertices instead of the labeled connector vertices. To find all possible synthon substructures, we have to consider all possible labeling patterns of the connector vertices. For a synthon reaction with x connector vertex types we end up with q x ( q !) labeled forests, because we have to consider all permutations of all connector vertex subsets of size q . The resulting forests of synthon substructures with labeled connector vertices then describe possible substructure hits in a synthon reaction: if we find an injective map from the computed synthon substructures to synthon sets of the synthon reaction, where these synthon sets contain synthons that contain the mapped synthon substructure, then we have a substructure hit. The full procedure that is performed for a specific cut set and a specific synthon reaction is summarized in Algorithm 2, where Ψ(F i , R) denotes the set of all injective maps from the connected components of the graph F i to the synthon sets of synthon reaction R. Please note that two important pruning steps will be explained in the next sections. for The connector region heuristic