CRISPR/Cas9 gene editing is a modern technique that targets specific stretches of genetic code and allows editing of deoxyribonucleic acid (DNA) at designated locations. It has been at the forefront of genetic research, providing scientists with the ability to undertake various genome-engineering projects, including opportunities for the modification of genes and the potential correction of mutations to prevent genetic diseases. However, the CRISPR (which stands for clustered regularly interspaced short palindromic repeats) genomic tool is limited by its precision. Specifically, the limiting factor is the DNA-cutting enzyme. The most commonly used enzyme is known as Cas9, but it only attaches to and targets a particular three-base sequence in the DNA that occurs in one-sixteenth of the human genome. This hampers the ability of researchers to carry out many applications that would entail editing of sequential segments of the DNA that occur more often. See also: CRISPR-based immunity in prokaryotes; CRISPR/Cas9 gene editing; CRISPR genome-editing methods against superbugs; Deoxyribonucleic acid (DNA); Enzyme; Gene; Genetic code; Genetic engineering; Genetics; Mutation

The CRISPR gene-editing system often utilizes an enzyme called Cas9 (blue) that cuts through double-stranded deoxyribonucleic acid (DNA; orange-red) at a specific sequential site. In addition, a ribonucleic acid (RNA) targeting device (purple) looks for the specific sequence of DNA to be cut (circled in black). Further manipulations allow researchers to cut and insert new sections of DNA in attempts to treat genetic causes of disease. (Credit: National Institute of General Medical Sciences, National Institutes of Health)

Investigators at Harvard University, under the direction of David Liu, have modified the Cas9 enzyme, now called xCas9, which targets a three-base sequence that occurs in one-fourth of the human genome. This greatly expands the range of possible gene-editing sites compared to the regularly used Cas9 enzyme, and more of the genome (four times as much) has the potential to be manipulated by researchers in various genomic operations. See also: Genomics; Human genetics; Human genome

Still, the xCas9 enzyme needs to be studied in greater detail. The commonly used Cas9 enzyme has been tested and utilized in thousands of studies, across the breadth of the human genome. In contrast, the xCas9 enzyme has been used in a small number of preliminary studies that have targeted only a few dozen sites on the genome. Thus, it is necessary to conduct further research to investigate whether xCas9 can be a more versatile DNA-cutting enzyme. In addition, xCas9 must be able to show the same precision as provided by Cas9, and it must not cause increased numbers of imprecise edits (called off-target mutations), which occur occasionally when using Cas9. If increased precision and expansion can be proven to be safe and reliable, then the CRISPR/xCas9 system will be embraced by the genetic research community. See also: Biotechnology