The I-SceI endonuclease that Jasin selected was one of the most specific endonucleases known at the time, requiring a perfect match of eighteen consecutive DNA letters for it to cut a given segment. Selecting a highly discriminating endonuclease was critical; if Jasin had chosen an enzyme that was too promiscuous, it would cut the genome all over the place, not only making the results more difficult to interpret, but potentially harming the host cell. With specificity for eighteen letters in a row, though, I-SceI would cut just one sequence of DNA out of the more than fifty billion possible
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