Hydroxymethylation enzyme defects have previously been associated with hematological malignancies; mutations in TET2 were found in acute myeloid leukemia (AML) or chronic myelomonocytic leukemia (CMML) and induced loss of hydroxymethylation and were linked with poor prognosis [27–30]. Here, TET2 is linked to chronic myelomonocytic leukemia.