DNA glycosylase that plays a key role in active DNA demethylation: specifically recognizes and binds 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC) in the context of CpG sites and mediates their excision through base-excision repair (BER) to install an unmethylated cytosine (PubMed:21817016). Cannot remove 5-hydroxymethylcytosine (5hmC). According to an alternative model, involved in DNA demethylation by mediating DNA glycolase activity toward 5-hydroxymethyluracil (5hmU) produced by deamination of 5hmC (PubMed:21722948). Also involved in DNA repair by acting as a thymine-DNA glycosylase that mediates correction of G/T mispairs to G/C pairs: in the DNA of higher eukaryotes, hydrolytic deamination of 5-methylcytosine to thymine leads to the formation of G/T mismatches. Its role in the repair of canonical base damage is however minor compared to its role in DNA demethylation. It is capable of hydrolyzing the carbon-nitrogen bond between the sugar-phosphate backbone of the DNA and a mispaired thymine. In addition to the G/T, it can remove thymine also from C/T and T/T mispairs in the order G/T >> C/T > T/T. It has no detectable activity on apyrimidinic sites and does not catalyze the removal of thymine from A/T pairs or from single-stranded DNA. It can also remove uracil and 5-bromouracil from mispairs with guanine. Hydrolyzes mismatched double-stranded DNA and polynucleotides, releasing free thymine. Homodimer. Interacts with AICDA and GADD45A (By similarity). Sumoylation on Lys-341 by either SUMO1 or SUMO2 induces dissociation of the product DNA. Embryonic lethality between 10.5 and 12.5 dpc. Embryos display specific patterning defects of the developing heart; vasculogenesis defects of dorsal aortae, carotid arteries and branchial arteries. Global defects of angiogenesis are also observed. Defects are due to epigenetic aberrations affecting the expression of developmental genes, coincident with imbalanced histone modification and CpG methylation at promoters of affected genes. Belongs to the uracil-DNA glycosylase (UDG) superfamily. TDG/mug family.