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 (PubMed:19372391, PubMed:21496894). Also mediates subsequent conversion of 5hmC into 5-formylcytosine (5fC), and conversion of 5fC to 5-carboxylcytosine (5caC) (PubMed:21778364). In addition to its role in DNA demethylation, plays a more general role in chromatin regulation by recruiting histone modifying protein complexes to alter histone marks and chromatin accessibility, leading to both activation and repression of gene expression (PubMed:33833093). Plays therefore a role in many biological processes and diseases, including stem cell maintenance, T and B-cell development, inflammation regulation, genomic imprinting, neural activity or DNA repair (PubMed:31278917). Involved in the balance between pluripotency and lineage commitment of cells it plays a role in embryonic stem cells maintenance and inner cell mass cell specification. Plays an important role in the tumorigenicity of glioblastoma cells. TET1-mediated production of 5hmC acts as a recruitment signal for the CHTOP-methylosome complex to selective sites on the chromosome, where it methylates H4R3 and activates the transcription of genes involved in glioblastomagenesis (PubMed:25284789). Binds preferentially to DNA containing cytidine-phosphate-guanosine (CpG) dinucleotides over CpH (H=A, T, and C), hemimethylated-CpG and hemimethylated-hydroxymethyl-CpG (PubMed:29276034). Plays an essential role in the protection and maintenance of transcriptional and developmental programs together with QSER1 to inhibit the binding of DNMT3A/3B and therefore de novo methylation (PubMed:33833093). 2-oxoglutarate + a 5-methyl-2'-deoxycytidine in DNA + O2 = a 5-hydroxymethyl-2'-deoxycytidine in DNA + CO2 + succinate 2-oxoglutarate + a 5-hydroxymethyl-2'-deoxycytidine in DNA + O2 = a 5-formyl-2'-deoxycytidine in DNA + CO2 + H2O + succinate 2-oxoglutarate + a 5-formyl-2'-deoxycytidine in DNA + O2 = a 5-carboxyl-2'-deoxycytidine in DNA + CO2 + H(+) + succinate Binds 1 Fe(2+) ion per subunit. Binds 3 zinc ions per subunit. The zinc ions have a structural role. Interacts with HCFC1 and OGT (By similarity). Interacts with SIN3A; recruits the transcriptional corepressor SIN3A to gene promoters (PubMed:21490601). Found in a complex composed of at least SINHCAF, SIN3A, HDAC1, SAP30, RBBP4, OGT and TET1 (By similarity). Interacts with QSER1 (PubMed:33833093). Interacts with NONO (via DNA-binding domain); this interaction recruits TET1 to genomic loci (By similarity). Expressed in fetal heart, lung and brain, and in adult skeletal muscle, thymus and ovary. Not detected in adult heart, lung or brain. Up-regulated in glioblastoma cells (at protein level) (PubMed:25284789). The CXXC zinc finger mediates binding to CpG-DNA. Glycosylated. Interaction with OGT leads to GlcNAcylation (By similarity). A chromosomal aberration involving TET1 may be a cause of acute leukemias (PubMed:12646957). Translocation t(10;11)(q22;q23) with KMT2A/MLL1. This is a rare chromosomal translocation 5' KMT2A/MLL1-TET1 3' (PubMed:12124344, PubMed:12646957). 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 (PubMed:21496894). According to another model, cytosine demethylation is rather mediated via conversion of 5hmC into 5fC and 5caC, followed by excision by TDG and replacement by unmethylated cytosine. Contaminating sequence. Potential poly-A sequence.