CRISPR knockout requires expression of both a single-guide RNA (sgRNA) that targets the specific site for knockout, and expression of the Cas9 nuclease which cuts the DNA at the target site. Also, for studies using the CRISPRa or CRISPRi systems, cells need to express the corresponding dCas9 activator (e.g. dCas9-VPH or dCas9-VPR) or dCas9-repressor (e.g. dCas9-KRAB). For many CRISPR applications then, it is useful or necessary to have cells stably expressing one of the the Cas9 proteins.