Meganuclease I-Sce I is a homing endonuclease. It recognises an 18-base pair sequence TAGGGATAACAGGGTAAT and leaves a 4 base pair 3' hydroxyl overhang. It is a rare cutting endonuclease. Statistically an 18-bp sequence will occur once in every 6.9*1010 base pairs (a frequency of 1 in 418). This sequence does not normally occur in a human or mouse genome.
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Meganuclease I-SceI Wikipedia(Text) CC BY-SA