Versatility of chemically synthesized guide RNAs for CRISPR-Cas9 genome editing

•CRISPR-Cas9 with synthetic crRNA:tracrRNA is an efficient system for gene editing.•Chemically synthesized guide RNAs allow for incorporation of chemical modifications.•Synthetic guide RNAs enable efficient high throughput arrayed screening.•DNA-free gene editing is achievable with synthetic guide RNAs and Cas9 mRNA/protein. The CRISPR-Cas9 system has become the most popular and efficient method for genome engineering in mammalian cells. The Streptococcus pyogenes Cas9 nuclease can function with two types of guide RNAs: the native dual crRNA and tracrRNA (crRNA:tracrRNA) or a chimeric single guide RNA (sgRNA). Although sgRNAs expressed from a DNA vector are predominant in the literature, guide RNAs can be rapidly generated by chemical synthesis and provide equivalent functionality in gene editing experiments. This review highlights the attributes and advantages of chemically synthesized guide RNAs including the incorporation of chemical modifications to enhance gene editing efficiencies in certain applications. The use of synthetic guide RNAs is also uniquely suited to genome-scale high throughput arrayed screening, particularly when using complex phenotypic assays for functional genomics studies. Finally, the use of synthetic guide RNAs along with DNA-free sources of Cas9 (mRNA or protein) allows for transient CRISPR-Cas9 presence in the cell, thereby resulting in a decreased probability of off-target events.