5-hydroxymethylcytosine (5-hmC) is a novel covalent modification of DNA, which is generated by the conversion of 5-mC to 5-hmC by the TET (Ten Eleven Translocation) family of proteins. TET2 and 5-hmC are important in the maintenance and differentiation of hematopoietic stem cells (HSCs) and are thought to play a role malignant transformation.