At the end of 2018, for the first time in history, the two Chinese twins Lulu and Nana were born with their genome modified by the CRISPR / Cas9 system. This outraged the entire scientific community and ask us about the risks of therapies targeting Human genome. The Cas9 nuclease is often compared to DNA scissors, allowing a very precise cut into any gene. CRISPR are the hands that position these scissors at the desired site of the genome. This system promises to be able to repair any defective gene, thus appearing as the solution for many genetic diseases. It is a valuable tool as well for many therapeutic approaches using genome engineering.