April 12, 2021 -- A new gene silencing tool allows scientists to switch genes on and off without altering genetic sequences. The tool, described in a paper published in Cell on April 9, uses a modified CRISPR-Cas9 system to introduce reversible epigenetic changes that control gene expression. The classic CRISPR-Cas9 system led to an explosion in gene editing studies since it was first developed almost a decade ago. It became quickly popular thanks to its ease of use compared to previous gene editing methods. CRISPR-Cas9 uses guide RNA to chaperone the Cas9 nuclease to a particular site in the DNA and create a double-strand break. By adding a specially designed short sequence of DNA, scientists can introduce permanent genetic changes to the cell. This oligonucleotide is used as a template by the cell when repairing the double-strand break, incorporating the sequence into the cell's genome. However, this approach can cause unintended genetic changes.