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Scientist He Jiankui showed the world how human embryo editing is relatively easy to do but incredibly difficult to do well. Going against the consensus in the scientific community that CRISPR-Cas9 technology is still too experimental and dangerous to use in human embryos, he applied it to forever change the genomes of twin girls to give them immunity to HIV.

His reckless experimentation on the girls in China not only shattered scientific, medical and ethical norms, it was also medically unnecessary: while the girls’ father is HIV-positive, it’s rare for fathers to pass on the virus to their children, and if they do, potent drugs can now control the infection.

Since 2012, when CRISPR-Cas9 transformed research, scientists have purposely taken a cautious and deliberate approach, focusing on how to safely apply genome editing to cure genetic diseases, fight cancer, accelerate drug development, create transplant organs and develop more nutritious crops. Its potential to improve our lives is enormous. But its potential to harm, with unintended side effects, is still unknown.

As the scientific community now works to establish stronger safeguards, He’s fateful decision to ignore the basic medical mantra of “do no harm” and risk the unintended consequences will likely be remembered as one of the most shocking misapplications of any scientific tool in our history.

Doudna is a co-inventor of CRISPR-Cas9, a professor at the University of California, Berkeley, and a co-author of A Crack in Creation