CRISPeR Frenzy asked Luigi Naldini of the San Raffaele Telethon Institute for Gene Therapy in Milan for comment on three studies published in June on the preprint server bioRxiv. The experiments were carried out independently by the groups of Kathy Niakan of the Francis Crick Institute in London, Dieter Egli of Columbia University in New York City, and Shoukhrat Mitalipov of Oregon Health & Science University in Portland. These findings heighten safety concerns about heritable genome editing (see the news item by Heidi Ledford in Nature). Below you can read Naldini’s thoughts.