TET1 is a key enzyme involved in DNA demethylation and gene regulation. It hydroxylates 5-methylcytosine (mC) to 5-hydroxymethylcytosine (hmC), which plays a role in maintaining DNA methylation fidelity. The study shows that TET1 binds to the genome of embryonic stem cells, particularly at transcription start sites (TSSs) of CpG-rich promoters and within genes. hmC is enriched in gene bodies and at CpG-rich TSSs, suggesting that TET1 helps regulate DNA methylation by converting mC to hmC. TET1 also binds to a significant proportion of Polycomb group target genes and associates with the SIN3A co-repressor complex, which is involved in transcriptional repression. The study indicates that TET1 fine-tunes transcription, opposes aberrant DNA methylation at CpG-rich sequences, and contributes to the regulation of DNA methylation fidelity. TET1's role in transcriptional repression is independent of its catalytic activity, and it interacts with the SIN3A complex to regulate gene expression. The findings highlight the importance of TET1 in maintaining proper DNA methylation patterns and gene regulation during development.TET1 is a key enzyme involved in DNA demethylation and gene regulation. It hydroxylates 5-methylcytosine (mC) to 5-hydroxymethylcytosine (hmC), which plays a role in maintaining DNA methylation fidelity. The study shows that TET1 binds to the genome of embryonic stem cells, particularly at transcription start sites (TSSs) of CpG-rich promoters and within genes. hmC is enriched in gene bodies and at CpG-rich TSSs, suggesting that TET1 helps regulate DNA methylation by converting mC to hmC. TET1 also binds to a significant proportion of Polycomb group target genes and associates with the SIN3A co-repressor complex, which is involved in transcriptional repression. The study indicates that TET1 fine-tunes transcription, opposes aberrant DNA methylation at CpG-rich sequences, and contributes to the regulation of DNA methylation fidelity. TET1's role in transcriptional repression is independent of its catalytic activity, and it interacts with the SIN3A complex to regulate gene expression. The findings highlight the importance of TET1 in maintaining proper DNA methylation patterns and gene regulation during development.