For the standard CRISPR system, knockout efficiency is directly dependent on Cas9 expression levels. Cell lines stably expressing Cas9 are required for CRISPR screening with pooled sgRNA libraries. Transient expression of the Cas9 is not sufficient for pooled screening. For CRISPR screening purposes, it is important to use cells that express Cas9 well. Further, Cas9 expression levels definitely vary across different cell types, depending on the promoter used to drive Cas9 expression and the number of Cas9 copies per cell
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