Working with DNA is a common base for almost all life science branches. The manipulation of genes is often the method of choice to get insight into cellular processes. Thus DNA-editing proteins which are able to bind and open DNA strands site-specifically are of great importance. There are several to choose from, and all have their advantages and disadvantages. The Cas9 endonuclease of the bacterial derived CRISPR-Cas9 immune system is one of them. In contrast to its alternatives – meganucleases, ZFNs, and TALENs – the Cas9 DNA-binding determinant is an RNA molecule, which is easy to design. Thus Cas9 is very flexible concerning target sequence choice. Therefore it is frequently used to mutate DNA, and to produce knock-out and knock-in mutants.