Dioxygenase that catalyzes the conversion of the modified genomic base 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC) and plays a key role in active DNA demethylation. Has a preference for 5-hydroxymethylcytosine in CpG motifs. Also mediates subsequent conversion of 5hmC into 5-formylcytosine (5fC), and conversion of 5fC to 5-carboxylcytosine (5caC). Conversion of 5mC into 5hmC, 5fC and 5caC probably constitutes the first step in cytosine demethylation. Methylation at the C5 position of cytosine bases is an epigenetic modification of the mammalian genome which plays an important role in transcriptional regulation. In addition to its role in DNA demethylation, also involved in the recruitment of the O-GlcNAc transferase OGT to CpG-rich transcription start sites of active genes, thereby promoting histone H2B GlcNAcylation by OGT. 2-oxoglutarate + a 5-methyl-2'-deoxycytidine in DNA + O2 = a 5-hydroxymethyl-2'-deoxycytidine in DNA + CO2 + succinate Binds 1 Fe(2+) ion per subunit. Binds 3 zinc ions per subunit. The zinc ions have a structural role. Interacts with HCFC1 (By similarity). Interacts with OGT. Highly expressed in the brain, kidney, heart, lung, muscle and stomach. Present in embryonic stem cells (ES cells). May be glycosylated. It is unclear whether interaction with OGT leads to GlcNAcylation. According to a report, it is GlcNAcylated by OGT (PubMed:23352454). In contrast, another group reports no GlcNAcylation by OGT in human ortholog. Mice are viable and fertile but develop chronic myelomonocytic leukemia probably caused by dysregulation of hematopoietic stem cells. Mice lacking both Tet1 and Tet2 are fertile, with females having smaller ovaries and reduced fertility. They display decreased 5-hydroxymethylcytosine (5hmC) and abnormal methylation at various imprinted loci. Embryonic stem cells lacking both Tet1 and Tet2 remain pluripotent but lack 5hmC, leading to developmental defects in chimeric embryos. Belongs to the TET family. Subsequent steps in cytosine demethylation are subject to discussion. According to a first model cytosine demethylation occurs through deamination of 5hmC into 5-hydroxymethyluracil (5hmU) and subsequent replacement by unmethylated cytosine by the base excision repair system. According to another model, cytosine demethylation is rather mediated via conversion of 5hmC into 5fC and 5caC, followed by excision by TDG (PubMed:21817016). Extended N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.