In Jinek et al., the authors show nuclease activity of their CRISPR/Cas9 system using the so-called Surveyor assay method. This assay recognizes small mismatches in dsDNA which are introduced by error prone non-homologuous end-joining after dsDNA breaks have been introduced by some foreign agent (such as Cas9). The Surveyor assay then -again- breaks the DNA at those points where it recognizes mismatches. The newly broken DNA fragments can be assayed and so one can identify the position of the initial, Cas9 dependent breakage.
My question is: for direct demonstration of Cas9 activity at the programmed position, why not first silence non-homolguous end-joining repair mechanisms in the cell and then directly analyse the lysate? Is this technically not feasible? Or is it not done in order to not introduce another change in the cell for which there would be no control?