Genome editing technologies have become valuable tools in many aspects of scientific research, facilitating, for example, the identification of genes involved in development, or the determination of protein functions. They work by creating breaks within the genome, which are then repaired by re-joining the ends of the DNA. Errors often occur during this process, potentially leading to loss-of-function mutations. Additionally, as both strands of the DNA are broken, it provides an opportunity to incorporate new DNA sequences into the genome.

The CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) / Cas (CRISPR-associated) system is one such method of genome editing. It has become difficult to ignore, having received a large amount of attention from both the scientific community and the media. One story that received significant coverage was the approval of a laboratory, based in London, to use CRISPR genome editing on human embryos. Several articles have also demonstrated the huge number of potential applications for this technique in animals, for example, producing cattle that do not develop horns and therefore avoid the need to undergo painful procedures to remove them. The reason for such excitement over CRISPR is, in part, due its simplicity and effectiveness.

The type II CRISPR system  requires only a crisprRNA (crRNA), a trans-activating crRNA (tracrRNA), and a Cas9 protein. The RNA sequences can be fused to create a single guideRNA (gRNA). This can be reprogrammed by changing a 20 bp region known as the spacer sequence, to target the Cas protein to almost any region within the genome, referred to as the protospacer. The Cas9 protein recognises a 3 bp recognition sequence within the genome, known as a protospacer adjacent motif (PAM). This allows it to bind and introduce a break within the DNA sequence, exactly 3 bp upstream of the PAM (Belhaj et al., 2013). Unlike with alternative techniques for introducing DNA breaks, such as TALENs, various genomic regions can be easily targeted by simply modifying the RNA sequences used to target the Cas protein.