Cas9, when combined with a piece of RNA to guide it, can be directed to make a deliberate change in the human genome. You can analogize it to finding and erasing one word in one sentence on one page in one volume of that eighty-thousand-book library. It errs occasionally and erases an unintended word as well, but its general fidelity is remarkable. More recently, the system has been modified not just to erase words but also to implement a vast array of potential changes in a gene, such as adding new information or making more subtle alterations. Cas9 is a search-and-destroy eraser. To continue
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