AbstractA recent report from Dr He Jiankui concerning the birth of twin girls harbouring mutations engineered by CRISPR/Cas nucleases has been met with international condemnation. Beside the serious ethical concerns, there are known technical risks associated with CRISPR/Cas gene editing which further raise questions about how these events could have been allowed to occur. Numerous studies have reported unexpected genomic mutation and mosaicism following the use of CRISPR/Cas nucleases, and it is currently unclear how prevalent these disadvantageous events are and how robust and sensitive the strategies to detect these unwanted events may be. Although Dr Jiankui’s study appears to have involved certain checks to ascertain these risks, the decision to implant the manipulated embryos, given these unknowns, must nonetheless be considered reckless. Here I review the technical concerns surrounding genome editing and consider the available data from Dr Jiankui in this context. Although the data remains unpublished, preventing a thorough assessment of what was performed, it seems clear that the rationale behind the undertaking was seriously flawed; the procedures involved substantial technical risks which, when added to the serious ethical concerns, fully justify the widespread criticism that the events have received.