&gt;&quot;No pre-existing cells have the mutation at the site you&#x27;re trying to engineer. It just doesn&#x27;t happen.&quot;<p>Not in any paper on CRISPR I have read, in fact just the opposite: there are always low levels of mutants found in the controls (eg Schumann et al 2015 linked below). Please link to the papers that have lead you to make this claim.