text: 5-Methylcytosine and 5-hydroxymethylcytosine both read as a C in bisulfite sequencing. Oxidative bisulfite sequencing is a method to discriminate between 5-methylcytosine and 5-hydroxymethylcytosine at single base resolution. The method employs a specific (Tet-assisted) chemical oxidation of 5-hydroxymethylcytosine to 5-formylcytosine, which subsequently converts to uracil during bisulfite treatment. The only base that then reads as a C is 5‑methylcytosine, giving a map of the true methylation status in the DNA sample. Levels of 5‑hydroxymethylcytosine can also be quantified by measuring the difference between bisulfite and oxidative bisulfite sequencing.