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In vertebrates oocytes undergo a series of maturation steps arresting at metaphase Ⅱ, and can then be fertilized by a sperm.Within a short period of time following fertilization, the paternal genome has been found to be actively demethylated, and this phenomenon has been proposed to be important for the epigenetic reprogramming of condensed spermchromosomes.In particular,trimethylation of histone 3 lysine 27 (H3K27me3) is associated with genespecific transcription repression and contributes to the maintenance of embryonic stem cell pluripotency.