DNA methylation plays an important role in epigenetic regulation of human gene expression. Mechanism of active demethylation of the human genome have been a matter of discussion for many years. Recently, a novel group of TET protein family enzymatically converting 5-methylcytosine into 5-hydroxymethylcytosine was discovered, playing a role in active DNA demethylation pathway. Results obtained in subsequent studies pointed that 5-hydroxymethylcytosine was not only an intermediate in that pathway, but might also modify epigenetic profile of the human genome.