RTase and GTase domains since the abolition of the RTase domain EX 527 HDAC inhibitor reduces HCE MZP susceptibility. In order to gain additional details on the MZP main binding site, molecular docking could be used. Unfortunately, the structure of both the RTase and the GTase domain are separately available, but the structure of the full-length protein is still not available. Nevertheless, we ran a molecular docking experiment of MZP on the GTase domain from HCE. The lack of RTase domain, which is predicted to participate in MZP binding, and the moderate flexibility of the N-terminal domain, which introduces a structural incertitude, does not allow for definitive conclusions to be reached but it is interesting to note that, in preliminary experiments, MZP favorably docks on the N-terminal region of the HCE GTase. Together, our results reveal that MZP inhibits the HCE GTase activity with a 5-to 25-fold specificity in comparison to other GTases. Although more work is yet required to confirm our hypothesis, these results raise the possibility that the GTase inhibition could be mediated by a conformational change hindrance upon binding of MZP to an allosteric binding site that is speculated to reside near the RTase-GTase inter-domain. Nevertheless, mizoribine is one of the first compounds to demonstrate a certain degree of specificity toward a single GTase, despite the high degree of conservation of this crucial family of enzyme. MZP displays a higher in vitro inhibition potency for the GTase reaction in comparison to the complete RNA capping reaction. This may simply be due to our experimental conditions where the RTase activity of HCE, which is partially inhibited by free Mg2+, becomes the rate-limiting step. However, in cellulo the RTase harbors a higher BEZ235 turnover rate than the GTase, which catalyzes the limiting step in RNA capping. In a cellular context we expect the efficiency of MZP to be dictated solely by its interaction with the GTase. Historically, very few GTase inhibitors have been developed, neither as scientific tools nor as therapeutic agents. More recently however, novel GTase inhibitors have been discovered. They include the allosteric inhibitor mycophenolic acid, the pyrophosphate analog foscarnet which acts as a product inhibitor, and ribavirin triphosphate, a GTP analog that is transferred to acceptor RNAs by GTase, leading to stable but inefficiently translated pseudo-capped RNA. The current study identifies MZP as a novel allosteric GTase inhibitor, which is speculated to block a crucial conformational change. The GTase activity being the ratelimiting step of the essential capping apparatus, all these GTase inhibitors are promising lead candidates for the development of novel selective capping inhibitors and lead the way to a new class of anti-cancer, antifungal, and antiviral drugs. What is the biological relevance of the present finding? Numerous studies have demonstrated the potency of MZP to inhibit the cellular IMPDH and to lower the intracellular guanosine nucleotide pool thereby limiting cell growth, but none have addressed its impact on the capping apparatus. Monitoring the capping efficiency in living cells is a great challenge as the cellular quality control machinery degrades unsuccessfully capped mRNAs. Since proper capping is crucial for mRNA transcription, export, stability and translation, it is possible to monitor the capping efficiency based on the translation of a reporter protein.