DNA strands containing a 5-hydroxymethylcytosine (^hmC), which have recently been found in neuron cells and embryonic stem cells, were synthesized through a facile synthetic technique. The ^hmC-containing strands were efficiently oxidized at ^hmC using an osmium oxidation assay. The ^hmC was oxidized as easily as 5-methylcytosine, which can be distinguished from unmethylated cytosine.