embryo
• only 3% of E0.5 embryos cultured for 4 days reach the blastocyst stage
• most embryos fragment or show abnormal cleavage at the 2-4 cell stage
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cellular
• methylation status of the maternal and paternal genomes of null zygotes is reduced compared to control zygotes
• however, methylation of oocytes before fertilization is similar to controls
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