A Guide to Guides: An Overview of SpCas9 sgRNA Scaffold Variants and Modifications

The CRISPR/SpCas9 system has revolutionized biology by enabling precise and programmable genome modification. While substantial effort has focused on engineering the SpCas9 protein and spacer sequences, the single-guide RNA (sgRNA) scaffold is an equally critical determinant of activity. Since the canonical scaffold was introduced in 2012, numerous variants have been developed. Early designs sought to enhance editing efficiency; however, despite the first improved scaffold being reported in 2013, more than 80% of CRISPR plasmids deposited in the Addgene repository still use the original scaffold rather than an efficiency-optimized alternative, which may not provide optimal performance. Subsequent work has also addressed intra-sgRNA interactions that impair folding, as well as inter-sgRNA interactions that destabilize multiplexed arrays, yet these solutions remain largely overlooked. Beyond efficiency, scaffold engineering—and the inclusion of auxiliary RNA elements—has enabled new capabilities, including effector recruitment, conditional regulation, visualization, improved stability, and large-scale multiplexing. The main goal of this review is to (i) provide a structured overview of the diverse SpCas9 sgRNA scaffold variants and auxiliary RNA modifications developed to date, (ii) summarize their functional characteristics and contexts of use, thereby illustrating how scaffold engineering continues to expand the functional scope of CRISPR technologies, and (iii) present a curated sequence resource comprising more than 230 scaffold variants and 80 auxiliary modifications to support experimental design and benchmarking.