https://phys.org/news/2018-10-crispr-tool-genome.html

However, despite the gene-editing tool's considerable success, CRISPR-Cas9 remains limited in the number of locations it can visit on the genome.

That is because CRISPR needs a specific sequence flanking the target location on the genome, known as a protospacer adjacent motif, or PAM, to allow it to recognize the site.