Anjana Ahuja, Nicolle Liu and Louise Lucas:

“Off-target effects” are when an edit intended for one location in the genome accidentally crops up elsewhere, like a stray comma. Such mistakes could litter the girls’ germlines in perpetuity. A single gene, moreover, might juggle several roles in a genomic balancing act that has evolved over millennia. This, experts say, is possible with CCR5: a mutation may protect against HIV but research suggests it may also increase vulnerability to West Nile virus and influenza. 

Given that the CCR5 mutation has been tentatively associated with improved brain function, Mr He’s self-funded handiwork might even come to be seen as an exercise in genetic enhancement.

Prof Collins says there is now an urgent need for a binding scientific consensus; otherwise “a technology with enormous promise for prevention and treatment of disease will be overshadowed by justifiable public outrage, fear, and disgust.”

Jennifer Doudna, the University of California Berkeley professor widely considered a founder of gene-editing and tipped for a Nobel Prize, said she was “horrified and stunned” by the development. Things may get worse: researchers examining Mr He’s work have suggested the enterprise might have been botched, with the girls carrying a mix of edited and unedited cells — something that would have unknown long-term effects.