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Genomic imprinting is an allele-specific gene expression system that is important for mammalian development and function. The molecular basis of genomic imprinting is allele-specific DNA methylation^,. Although it is well known that the de novo DNA methyltransferases Dnmt3a and Dnmt3b are responsible for the establishment of genomic imprinting, how the methylation mark is erased during primordial germ cell (PGC) reprogramming remains unclear. Tet1 is one of the ten-eleven translocation family proteins, which have the capacity to oxidize 5-methylcytosine (5mC)^,,, specifically expressed in reprogramming PGCs. Here we report that Tet1 has a critical role in the erasure of genomic imprinting. We show that despite their identical genotype, progenies derived from mating between Tet1 knockout males and wild-Peg10 and Peg3, which exhibit aberrant hypermethylation in the paternal allele of differential …