Reaction mechanisms are oftentimes not only influenced by reaction vertices and edges but also by adjacent structural components such as functional groups that determine the local chemical environment. It is desirable, therefore, to consider patterns L in DPO rules that also include features that are not altered during the reaction. Formally this can be achieved by considering subgraphs Q of the ITS Υ(G, H, α) containing the reaction center as a proper subgraph, Γ ⊂ Q ⊆ Υ(G, H, α). The choice of the graph Q is the difficult issue here.