5-Hydroxymethylcytosine in the mammalian zygote is linked with epigenetic reprogramming. 5-Hydroxymethylcytosine is a DNA pyrimidine nitrogen base formed from the enzymatic conversion of 5-methylcytosine into 5-hydroxymethylcytosine by the TET family of iron-dependent oxygenases. Data suggests that every mammilian cell contains 5-hydroxymethylcytosine, but the levels vary depending on the cell type; data also suggests that levels of 5-hydroxymethylcytosine increases with age. The highest levels are found in neuronal cells of the central nervous system and certain mammalian tissues such as mouse Purkinje and granule neurons. Although the exact function has not been fully elucidated, studies suggest that 5-hydroxymethlcytosine may regulate gene expression or initiate DNA demethylation.