Illustration showing the action of Cas9 proteins (blue) from a Streptococcus pyogenes bacterium (SpCas9). SpCas9 is an endonuclease enzyme which splices DNA (deoxyribonucleic acid) as a defence mechanism against bacteriophages (viruses). It does so by binding to guide RNA (ribonucleic acid, pink) which is complementary to the DNA (yellow/orange) sequence which is to be spliced. This mechanism makes SpCas9 an effective tool in gene-editing technology. DNA can be spliced at desired locations by synthesising guide RNA, after which genes can be removed and/or added to an organism?™s genome.