5-hydroxymethylcytosine (5hmC) is a recently discovered epigenetic modification of DNA, sometimes known as the ‘sixth base’. The function of 5hmC is still only poorly understood, but it is thought to play a role in transcriptional activation and DNA demethylation. Immunohistological and HPLC studies have shown that 5hmC becomes depleted in a wide range of cancers, including brain tumours1. However, no information yet exists on whether this reduction in 5hmC in brain tumours is global or locus-specific; furthermore, the reason and mechanism behind this decrease remains unclear. It has been speculated that the loss of 5hmC in cancer may be caused by mutations in isocitrate dehydrogenase (IDH), a gene which acts upstream of the ten-eleven translocation (TET) family of enzymes and is frequently mutated in brain tumours2 . We wish to investigate the changes in 5hmC that occur in glioblastoma (GBM) and whether they are linked to IDH mutation. GBM is the most common and aggressive form of adult primary brain tumour; they are highly invasive, impossible to completely resect, and patients have an extremely poor prognosis with no effective cure and a median survival time of less than one year. We used oxidative reduced representation bisulfite sequencing to map 5hmC at single-base resolution in 6 IDH-mutant and 6 IDH-wild type GBMs. This study is the first investigation of 5hmC changes in brain tumours at a locus-specific level, and has the potential to reveal new insights into the early underlying mutations and epigenetic perturbations responsible for GBM, as well as identifying new epigenetic markers for patient survival.