Updated project metadata. Surveillance of DNA methylation in mammals is critical for genome stability and epigenetic regulation. The discovery of the ten-eleven translocation (TET) proteins catalyzing the oxidation from 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC) revolutionized the understanding of DNA methylation dynamics. Interestingly, in recent years evidence accumulated that TET1 also harbours non-catalytic functions. However, the role and mechanism of TET1 DNA demethylation independent functions still remain poorly understood. Here, we use genome engineering and quantitative multi-omics approaches to dissect the non-catalytic role of TET1. Strikingly, we find that the majority of transcriptional regulation depends on non-catalytic functions of TET1. To gain insights into possible mechanisms by which TET1 regulates transcription independent of DNA demethylation, we asked if the loss of TET1 is accompanied by changes in the histone modificaiton landscape. To this end, we compared the relative abundances of core histone modifications between Tet1 KO, Tet1 CM and WT mESCs using quantitative LC-MS/MS analysis. Surprisingly, we observed a profound global reduction of pH4Kac and H4K20me3 as well as H3K27me3 in Tet1 KO mESC. Vice versa, the monomethylation states of the latter two residues, H3K27me1 and H4K20me1 were significantly increased in Tet1 KO. Similar to the results from the transcriptome data, most of these changes were specific to Tet1 KO cells.