How do you know that HAP1 knockout cells don’t contain off-target modifications?
While we cannot exclude entirely the possibility of off target modifications in addition to on target cleavage, we believe it represents a relatively low and controllable risk, as a number of recent publications have demonstrated that the CRISPR-Cas9 system is to be highly specific (e. g. Cencic et al). Further to this a collaborator group have published a paper where they examine off-targets in the HAP1 cell line and observe very low frequencies. https: //www. nature. com/articles/nmeth. 3284At Horizon, to mitigate the risk still further our in-house selection algorithms warrant that only those guide RNAs with minimal predicted off-target sites in the human genome are chosen. Further to this, scientists can control their experiments through the use of multiple, independent clones or rescue of the knockout with a wild-type cDNA.