[Enhancement can do harm]

Inactivation of the CCR5 gene by CRISPR editing in human embryos, as recently attempted in China, was touted as a positive change for the babies involved since it was expected to impart resistance to HIV infection. However, it turns out that the absence of CCR5 is not neutral but actually decreases fitness, as shown by survival analysis of population data in the UK biobank. This underlines the pitfalls of genetic enhancement, and emphasizes that any germline modification must be preceded by in-depth studies to exclude unforeseen negative effects. ‡.