Cells originating from the same population were trasfected in order to over-express either the active HCE-WT-HA, the GTase-defective HCE-K294A-HA mutant, the GFP control protein or no protein. They were submitted to concentrations of 0 mM, 40 mM or 120 mM of mizoribine. All cell lines treated with mizoribine showed a global reduction in reporter protein expression when compared with untreated cells. This expected effect is likely due to partial guanosine pool depletion induced by IMPDH inhibition. Interestingly, the reduction in transcription and translation of the reporter was significantly less severe only in cells over-expression HCE-WT-HA for both mizoribine concentrations. The ability of HCE-WT-HA over-expression to partially rescue the luciferase expression in the presence of mizoribine ABT-199 demonstrates that HCE is one of the mizoribine pharmacological targets. Furthermore, the inability of the GTase defective mutant HCE-K294A-HA to rescue the reporter expression under mizoribine treatment further demonstrates, in agreement with our in vitro results, that in a cellular context it is the GTase activity of HCE that is targeted by MZP. Although indirect, this is strong evidence that mizoribine is able to impair capping in a cellular environment. As expected, capping could not be fully inhibited in cellulo at mizoribine concentrations of 40�C120 mM, which is approximately the in vitro IC50 of 80 mM. Despite its oral bioavailability and its low binding to serum proteins, mizoribine was not expected to reach intracellular concentration higher then its IC50. Of notice, the effect of mizoribine on cellular capping was however greater than anticipated. We hypothesize that the competition between HCE and Xrn2 for the nascent mRNA 59 end could explain the potency of MZP in cellulo, as slowing the capping activity could be sufficient to shift the balance towards quality control take-over and reduce downstream protein expression. What is the exact contribution of the capping apparatus inhibition to the global mizoribine mechanism of action? This question has yet to be addressed, but the immunosuppressive effect of mizoribine on T-cell is mainly mediated by GTP depletion and might be exacerbated by the reduction of mRNA capping and downstream cap-dependent translation. Our study clearly demonstrates that the therapeutic agent mizoribine monophosphate SB431542 301836-41-9 inhibits the human RNA guanylyltransferase in vitro and impairs mRNA capping in cellulo. Estrogen receptor negative breast cancer types are generally more aggressive and prone to metastasize. The absence of Estrogen receptor-alpha correlates with hormone-independent growth of these mammary tumor cells and causes uncontrolled proliferation and insensitivity to anti-hormonal treatments. In ERa-negative cell lines, a subset of genes is epigenetically silenced, while the majority of genes involved in cell cycle control and proliferation are constitutively expressed. Aberrant gene expression is frequently the result of chromatin modifications and composition, including histone posttranslational modifications and/or incorporation of histone variants. In particular, deregulation of enzyme complexes responsible for histone acetylation and deacetylation can be associated with breast cancer progression and an increase in tumor malignancy. Thus, compounds that change chromatin modifications are a promising anti-cancer approach.