Multiple studies have found negative correlations between 5hmC levels and glioma grade16,17 and loss of 5hmC correlates with poor prognosis of GBM patients.13,18 This 5hmC loss is partially explained by isocitrate dehydrogenase (IDH1/2) mutations, which result in the production of 2-hydroxyglutarate (2HG) that inhibits TET enzymatic function.19 Interestingly, GBM are also characterized by low levels of 5hmC despite being predominantly IDH wild-type,20 suggesting a different mode of TET inactivation in these tumors. This evidence concerns the gene IDH1 and glioblastoma.