meiosis is a germ cell - specific cell division process through which haploid gametes are produced for sexual reproduction1 . prior to initiation of meiosis , 
 mouse primordial germ cells ( pgcs ) undergo a series of epigenetic reprogramming steps2,3 , including global erasure of dna methylation on the 5-position of cytosine ( 5mc ) at cpg4,5 . 
 although several epigenetic regulators , such as dnmt3l , histone methyltransferases g9a and prdm9 , have been reported to be critical for meiosis6 , little is known about how the expression of meiotic genes is regulated and how their expression contributes to normal meiosis . using a loss of function approach , here 
 we demonstrate that the 5mc - specific dioxygenase tet1 plays an important role in regulating meiosis in mouse oocytes . 
 tet1 deficiency significantly reduces female germ cell numbers and fertility . 
 univalent chromosomes and unresolved dna double strand breaks are also observed in tet1-deficient oocytes . 
 tet1 deficiency does not greatly affect the genome - wide demethylation that takes place in pgcs but leads to defective dna demethylation and decreased expression of a subset of meiotic genes . 
 our study thus establishes a function for tet1 in meiosis and meiotic gene activation in female germ cells .