The TET enzymes are a family of ten-eleven translocation (TET) methylcytosine dioxygenases. They are instrumental in DNA demethylation.  5-Methylcytosine (see first Figure) is a methylated form of the DNA base cytosine (C) that often regulates gene transcription and has several other functions in the genome.  
Demethylation by TET enzymes (see second Figure), can alter the regulation of transcription.  The TET enzymes catalyze the hydroxylation of DNA 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), and can further catalyse oxidation of 5hmC to 5-formylcytosine (5fC) and then to 5-carboxycytosine (5caC).  5fC and 5caC can be removed from the DNA base sequence by base excision repair and replaced by cytosine in the base sequence.
TET enzymes have central roles in DNA demethylation required during embryogenesis, gametogenesis, memory, learning, addiction and pain perception.