Outrage continues to grow over the news that Chinese scientist He Jiankui claims to have used gene-editing technology called CRISPR-Cas9 to create embryos that he then transferred into a woman, resulting in the birth of twin girls.

His stated goal was to give these girls genetic immunity to HIV, but experts say that he unnecessarily put their lives in danger by using a tool that could cause unwanted and unforeseen alterations in other parts of their genetic code, in ways that could be passed down to the girls future descendants.

Word of his experiment spread just as the worlds top gene-editing scientists gathered, along with He, in Hong Kong for the second international summit on human gene editing, to discuss, among other things, the thorny ethical implications of the technology. A committee of more than a dozen of those scientists released a consensus statement condemning Hes deeply disturbing news . They also called for an independent review to assess exactly what he and his team had done.

The 2018 consensus statement also affirmed the summits initial stance, first published in 2015, that genetic editing on sperm, egg or embryo cells that go on to become pregnancies is still too risky to attempt and that there are too many unknowns.

What Is CRISPR-Cas9?

CRISPR-Cas9 forms the basis of a tool used to make genetic alterations in DNA. It is not the worlds first gene-editing tool, but it is the cheapest, easiest, most accessible and most accurate one to date.

CRISPR-Cas9 itself is a component of the immune systems of bacteria. CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats, which is a mouthful of a term scientists use to describe particular types of DNA sequences present in bacterial genomes. These sequences act as genetic memory banks that bacteria use to protect themselves from invaders like viruses.

The first time a virus invades a bacterium, the bacterium stores a snippet of the viruss genome in its own and transcribes it into strands of RNA that will recognize the viral DNA if it invades again. In that case, those RNA strands direct special enzymes called CRISPR-assisted proteins (Cas) to the known DNA snippet in the invading virus. The enzyme goes to work to chop up the DNA, deactivating the threat.

Genetic scientists harness this seek-and-destroy mechanism to make targeted changes in DNA. MIT scientist Feng Zhang, who was integral in developing CRISPR-Cas9 applications for use in plant, animal and human cells, compares CRISPR-Cas9 to a tool that can find and replace typos in a Word document. First, CRISPR finds the mistake that needs to be fixed, and then the enzyme Cas9 cuts it out and replaces it with a new word.

To use CRISPR-Cas9 to edit a strand of DNA in a cell or animal, a scientist first creates a small circle of DNA that includes the genetic code for bacterial Cas9 and instructions for the cell to produce many copies of the enzyme. She inserts that circle of DNA, called an expression plasmid, into the cell, along with another plasmid loaded with information about the genetic sequence she wants the Cas9 protein to find in the genome. This second plasmid will be transcribed into strands of RNA, which will guide the Cas9 enzymes to the target gene in the cells DNA. Together, they find the targeted gene sequence and snip it out.

Depending on the scientists goals, she can either hope the cell takes it upon itself to repair the snipped DNA (leaving a double helix devoid of the targeted gene sequence), or she can inject a new strand of DNA that she wants the cell to use in place of the snipped DNA.

The uses for this genetic tool are practically limitless, said Josephine Johnston, director of research at The Hastings Center, a bioethics research institute.

Its potential applications are gigantic, said Johnston. This is something that can be used in bacteria or fish or mosquitoes or farm animals or humans.

Take wildlife conservation. Invading predator animals like rodents and possums have decimated New Zealands vibrant and diverse local bird species. Conservationists in that country are excited about the possibility of using CRISPR-Cas9 to introduce a genetic code into the rat population that would make it more difficult for them to reproduce, thereby driving down their numbers and giving the birds a chance to repopulate.

Public health researchers also wonder if they can use CRISPR-Cas9 to make it more difficult for mosquitoes to spread devastating diseases like malaria or Zika virus to human beings, either by making mosquitoes resistant to the disease itself or by simply altering their genetic code so that they stop reproducing altogether .

But with wide-ranging possibilities come serious concerns.