I have never seen a paper on CRISPR that can distinguish between selecting pre-existing mutants and actually modifying genes. I have read probably a dozen or so at this point, and it is amazing that they always fail to address this either in citations or actual data. At first I thought it was an honest mistake, but now it would not surprise me if some of the main players know that their experiments with CRISPR have b…
But there are plenty of knock-in experiments where foreign DNA was put into the cut site. Are you saying a pre-existing mutant with the foreign DNA was used in those cases too?
"Although rare (∼1–2%), edits were detected with Cas9-only control treatment, including at the predicted CXCR4 cut site, potentially indicating trace amounts of experimental contamination of the Cas9 RNPs." http://www.pnas.org/content/112/33/10437.full
Note the curiously missing rows in dataset S1.