Methylcytosine dioxygenases (encoded by TET1–3) are known to be critical enzymes for the oxidation of 5-methylcytosine residues within the RNA to generate 5-hydroxy-methylcytosine, and thereby play a key role for the regulation of gene expression in leukaemias which then drives leukaemogenesis and progression of malignant hematopoietic stem cells (HSCs).2 Albeit TET2 is considered to belong to the group of tumour-suppressor genes, mutated TET2 (only TET2 has been found to be altered in leukaemias) is known to be a key driver for leukaemogenesis. This evidence concerns the gene TET1 and leukemia.