Methylation of cytosine bases in DNA (5mC) represents a major epigenetic mark found in vertebrates genomes associated with transcriptional silencing1. 5mC is being introduced and maintained by DNA methyltransferases2-5, and has been shown to play important roles in a number of biological processes including genomic imprinting, X-chromosome inactivation, cellular differentiation, and development3, 6. Consequently, the disruption of 5mC genomic patterns is associated with a number of diseases7, 8-11. Despite the progress in understanding 5mC role in development and disease, it is still remain largely unknown how this mark is being removed in developing and adult tissues. Several potential mechanisms of DNA demethylation have been recently proposed including active and passive demethylation mechanisms12, 13, 14, 15. Discovery of the products of 5mC sequential oxidation mediated by ten-eleven translocation enzymes (tet1/2/3) such as 5-hydroxylmethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC) in eukaryotic DNA 16, 17, 18, 19 prompted speculations whether they may serve as intermediates of DNA demethylation processes or act as stable epigenetic marks in their own right13. While the demonstration that the component of base excision repair, thymine-DNA glycosylase (TDG) can bind and remove both 5fC and 5caC from DNA19, 20 suggests a role for the modified 5mC derivatives in an active DNA demethylation. Recent evidence showing that 5fC/5caC can modulate the rate of RNA II processivity points to potential involvement of these marks in transcriptional regulation29. Due to this potential biological importance of oxidized forms of 5mC, a range of biochemical and antibody based techniques have been employed for studying their genomic distribution and global content16, 19-24.