With gene body 5hmC associated with increased expression, and the trend towards accumulation of 5hmC observed in IDH1 mt tumors, an intriguing possibility exists that epigenetic dysregulation of gene expression in IDH1 mt gliomas arises, in part, due to impaired oxidation of 5hmC to 5fC across gene bodies, facilitating expression of target genes. The gene discussed is IDH1; the disease is central nervous system cancer.