DNA demethylation is a critical process in genome regulation, involving the removal of methyl groups from 5-methylcytosine (5mC). Recent discoveries have revealed that TET enzymes can oxidize 5mC to 5-hydroxymethylcytosine (5hmC), a key intermediate in active demethylation. This oxidation can be further processed through iterative oxidation to 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). Thymine DNA glycosylase (TDG) then excises these oxidized bases, facilitating the restoration of unmodified cytosine. This pathway represents a complete cycle of dynamic cytosine modification, essential for various biological processes. TET enzymes, part of the TET family, are crucial for active DNA demethylation. They oxidize 5mC to 5hmC, which can be passively diluted during DNA replication or actively repaired by TDG. The discovery of TET enzymes has significantly advanced our understanding of DNA demethylation mechanisms. TDG plays a vital role in excising modified bases, contributing to the active demethylation process. The interplay between TET and TDG ensures the proper regulation of DNA methylation, which is essential for development, gene regulation, and genome stability. Active DNA demethylation involves both enzymatic processes and DNA repair mechanisms. TET-mediated oxidation and TDG-mediated excision are key steps in this process. These mechanisms are crucial for cellular reprogramming, such as in the development of primordial germ cells and the generation of induced pluripotent stem cells. Additionally, DNA demethylation is involved in various biological contexts, including cancer, where aberrant methylation patterns can lead to disease. The study of DNA demethylation has revealed the importance of dynamic cytosine modifications in maintaining genome stability and responding to environmental changes. The roles of TET and TDG in this process highlight the complexity of DNA methylation and demethylation, with implications for both normal development and pathological conditions. Further research is needed to fully understand the mechanisms and functions of these processes, as well as their potential therapeutic applications.DNA demethylation is a critical process in genome regulation, involving the removal of methyl groups from 5-methylcytosine (5mC). Recent discoveries have revealed that TET enzymes can oxidize 5mC to 5-hydroxymethylcytosine (5hmC), a key intermediate in active demethylation. This oxidation can be further processed through iterative oxidation to 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). Thymine DNA glycosylase (TDG) then excises these oxidized bases, facilitating the restoration of unmodified cytosine. This pathway represents a complete cycle of dynamic cytosine modification, essential for various biological processes. TET enzymes, part of the TET family, are crucial for active DNA demethylation. They oxidize 5mC to 5hmC, which can be passively diluted during DNA replication or actively repaired by TDG. The discovery of TET enzymes has significantly advanced our understanding of DNA demethylation mechanisms. TDG plays a vital role in excising modified bases, contributing to the active demethylation process. The interplay between TET and TDG ensures the proper regulation of DNA methylation, which is essential for development, gene regulation, and genome stability. Active DNA demethylation involves both enzymatic processes and DNA repair mechanisms. TET-mediated oxidation and TDG-mediated excision are key steps in this process. These mechanisms are crucial for cellular reprogramming, such as in the development of primordial germ cells and the generation of induced pluripotent stem cells. Additionally, DNA demethylation is involved in various biological contexts, including cancer, where aberrant methylation patterns can lead to disease. The study of DNA demethylation has revealed the importance of dynamic cytosine modifications in maintaining genome stability and responding to environmental changes. The roles of TET and TDG in this process highlight the complexity of DNA methylation and demethylation, with implications for both normal development and pathological conditions. Further research is needed to fully understand the mechanisms and functions of these processes, as well as their potential therapeutic applications.