Mammalian development begins with a single cell resulted from the fertilization of a sperm and an oocyte.The early embryonic genome undergoes profound epigenetic reprogramming to prepare for development.The biological significance and mechanisms of epigenetic reprogramming are poorly understood.We find that 5-methylcytosine (5mC), the most abundant type of base modification in DNA, is oxidized to 5-hydroxymethylcytosine (5hmC) as well as 5-carboxymethylcytosine (5caC) in mouse zygotes.In vitro, the Tet family of dioxygenases oxidize 5mC to 5caC under physiologically relevant conditions.In zygotes, the Ten-eleven-translocation protein Tet3 is responsible for the genome-wide oxidation of 5mC to 5hmC and 5caC.Deficiency of zygotic Tet3 impedes demethylation at the paternal Oct4 and Nanog genes and delays the reactivation of Oct4 in early embryos.The heterozygous mutant embryos lacking maternal Tet3 suffer increased developmental failures.Importantly, oocytes lacking Tet3 also show impaired reprogramming of injected somatic cell nuclei.In addition, MEFs deficient in all Tet genes were unable to be reprogrammed by Yamanaka factors.We conclude that Tet-mediated oxidation is important for DNA demethylation and gene activation in the early embryo following natural fertilization, as well as for the reprogramming in somatic cell nuclear transfer and factor-based iPSC generation.