actively demethylated was for many years a central question in the quest to better understand epigenetic regulation. Until recently, however, it remained unsolved, with a history of failures in the discovery of DNA demethylases in animal cells [1]. In 2009, the ten-eleven translocation 1 (TET1) gene was shown to encode a dioxygenase that converts 5-mC to 5-hydroxymethylcytocine (5-hmC) by transferring an oxygen atom to the methyl group of 5-mC [2]. This discovery triggered the subsequent rapid progress in understanding the mechanisms that underlie demethylation processes [3,4]. TET2 and TET3 were soon found to be products of genes belonging to the same family as TET1 and to exhibit similar dioxygenase functions [5,6]. Once a TET dioxygenase catalyzes the 5-mC to 5-hmC conversion, multistep biochemical reactions follow, and the position is ultimately replaced by the unmodified cytosine (uC) (Fig. 1).Somatic TET2 mutations are frequently identified in a wide variety of hematologic malignancies [7,8]. These mutations cause impairment of enzymatic activity of the TET2 dioxygenase, resulting in the failure of 5-mC to 5-hmC conversion, and eventually failure of demethylation. The abnormal 5-mC/5-hmC/uC pattern in the TET2 mutation-carrying cells is thought to cause changes in the expression of the target genes and eventually drive the cells to develop hematologic malignancies. However, it remains unclear how TET2 mutations change the scheme of methyl group modification of the cytosine at the CpG sites throughout the genome and how such structural changes affect gene expression profiles.There have, however, been important advances in our understanding of why TET2 mutations are identified so commonly in such a wide variety of hematologic malignancies and the biological significance of these mutations. In this PIH article, I will focus mainly on the biological Abstract The TET dioxygenases, TET1, TET2, and TET3, catalyze transfer of an oxygen atom to the methyl group of 5-methylcytocine (5-mC), converting it to 5-hydroxymethylcytocine (5-hmC). Among the genes encoding these enzymes, ten-eleven translocation 2 (TET2) is frequently mutated somatically in both myeloid and lymphoid malignancies. Because these TET2 mutations result in the impairment of the dioxygenase activity of TET2, it is thought that these mutations interfere with 5-mC to 5-hmC conversion. There is ample evidence indicating that TET2 mutations are a driver of tumorigenesis in blood cells and that TET2 mutations are often acquired at the hematopoietic stem/early progenitor cell stage. In addition, TET2 is the second-most frequently mutated gene in clonal hematopoiesis in individuals with no apparent blood cancers, suggesting that while TET2 mutations alone are insufficient to cause hematologic malignancy, they represent an early event during tumorigenesis. A number of questions, including the precise target genome regions of TET2, and the importance of the balance of 5-mC and 5-hmC in the regulatory regions in transcriptional control, remain.