The molecular docking program DOCK (Version 4.0) was used to perform the virtual screening [27] . Componds displaying at least 80% of structural similarity to GTP or ATP (.25,000 compounds) were screened for the binding to the four GTases. The levels of similarities were measured using the Tanimoto equation [28] and the PubChem dictionary-based binary fingerprint (http:// pubchem.ncbi.nlm.nih.gov/). Based on the binding models of these compounds predicted by DOCK, the X-SCORE program (Version 1.1) was applied to obtain an estimate of the binding affinities of these compounds [29] . The compounds were then ranked according to their binding affinities as estimated by X-SCORE.