A team in China has taken a new approach to fixing disease genes in human embryos. The researchers created cloned embryos with a genetic mutation for a potentially fatal blood disorder, and then precisely corrected the DNA to show how the condition might be prevented at the earliest stages of development.

The team is also the first to edit out the mutation responsible for a ‘recessive’ disease: one caused by having two faulty copies of a gene. Because it would be difficult for researchers to find dozens of embryos that all have this rare double mutation, the team worked around this roadblock by developing embryonic clones from their patient’s skin cells.