Recent improvements in genome editing have vastly increased our ability to introduce such delicate defined mutations within the genomes of human cells. These are based on designer site-specific nucleases that introduce a double-strand break (DSB) at a desired site in the genome, which is then repaired by the cell, and can be utilised to introduce a variety of different genetic changes at this site. Most recent work focuses on the clustered, regularly interspaced short palindromic repeats (CRISPR) system and the use of the RNA-guided CRISPR-associated 9 (Cas9) endonuclease (Cho et al. 2013; Cong et al. 2013; Jinek et al. 2012, 2013; Mali et al. 2013), predominantly due to the simplicity by which this can be reprogrammed to bind to millions of sites within the genome. This system relies on a short guide RNA molecule to direct its specificity, through base pairing of its first 20 nt with the corresponding DNA sequence in the genome (Jinek et al. 2013). The only limitation to the targeting is a requirement for a protospacer adjacent motif (PAM) sequence adjacent to the target site, which
