A scientific research team has discovered might be considered emergency replacements for the epigenetic 'sentries' that normally protect the genome from transposons and viruses. These shock troops are pressed into service across the genome only during curiously undefended moments when early, preimplantation embryos are stripped of epigenetic marks and later reprogrammed. It could be one of the earliest forms of genome defense, created by snipping 18- and 22-nucleotide fragments from tRNAs.
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