The majority of the TbCet1 inhibitors that we identified showed strong specificity for the trypanosome enzyme and were manyfold less active (or inactive in some cases) against the orthologous TTM-type RTPases of mimivirus, baculovirus, and budding yeast (or the homologous bacterial phosphohydrolase CthTTM). Thus, (i) potent inhibition of a particular RTPase is no guarantee that the inhibitory compounds will have a quantitatively similar effect on another member of the TTM family, and (ii) an optimal search for inhibitors of the TTM-type mRNA cap-  ping enzymes of infectious pathogens will entail screening against RTPases from each organism or virus of interest. From the **data** in Table 5, we can infer certain trends in the responsiveness of other TTM enzymes to TbCet1 inhibitors. For example, SceCet1 was the least responsive, insofar as 13/25 TbCet1 inhibitors had no effect on the yeast enzyme (IC 50 , Ͼ500 M). MimiCE was unaffected by 7/25 inhibitors. CthTTM was also unaffected by 7/25 inhibitors, six of which overlapped with the compounds inactive against MimiCE. Lef4 was unaffected by 5/25 inhibitors, all five of which were also inactive versus MimiCE and CthTTM.