&gt;&quot;It sounds like you doubt the dsDNA nuclease activity of Cas9.&quot;<p>Not at all. This would be why the treatment is toxic and suppressive of proliferation.<p>At this point, I still think the presence of indels at the site (ie the proposed NHEJ mechanism) is just as easily explained by selection for pre-existing mutants. The experiments involving insertion of DNA (ie the proposed HDR mechanism) are better, but lack controls for &quot;off-target&quot; PCR amplification when showing the gels. IE we need to know how often the template itself will be amplified under their primers&#x2F;conditions, both free and if it gets incorporated in some random location.<p>When segments across the insertion junction are amplified, sequenced, and reported, I find this convincing as it is a precise prediction that matches the data and I can think of no other explanation. The other experiments are pretty much redundant and add nothing. However, the reports I have seen contain little methodological or quantitative information regarding these sequences which does make me remain skeptical, especially about claims of efficiency. Those claims seem to always be determined using the former experiments that can be explained in other ways.