Recently three new nucleobases were discovered in DNA isolated from mouse embryonic stem cells. The three nucleobases are oxidation products of 5-methylcytosine (mC), which is a base that regulates transcriptional activity. The oxidation reaction of 5-methylcytosine to 5-hydroxymethylcytosine (hmC), 5-formylcytosine (fC), and finally 5-carboxycytosine (caC) is now understood to be performed by 10-11-translocon proteins (TET1-3). These enzymes are R-ketoglutarate dependent oxidases, which directly utilize molecular oxygen for the oxidation reaction. The three new nucleobases are currently thought to be involved in epigenetic programming of cells, and they could be intermediates of a long searched for pathway of active demethylation. Recently new sequencing methods that allow the genome wide localization of hmC and fC in genomic DNA were reported.