He Jiankui says it is now possible to rewrite the very blueprint of life.
Yet many scientists say the experiment was premature and the potential benefits not worth the risk. He's experiment altered the genomes of embryos produced through IVF; their genetic changes will therefore be passed on to any future generations.
He said his goal was to give the babies a natural ability to resist HIV. HIV is responsible for AIDS and a cure for the disease is yet to be found.
Musunuru is a gene editing expert and editor of a genetics journal.
He told the AP he felt a strong responsibility "not just to make a first, but also to make an example" for future research.
"We advocate the establishment of a global observatory for gene editing, as a crucial step to determining how the potential of science can be better steered by the values and priorities of society", wrote Sheila Jasanoff and J. Benjamin Hurlbut in Nature.
"Regardless of where it was conducted, this work as described in press reports violates scientific conduct guidelines and is inconsistent with ethical norms of the scientific community and Rice University", the school said in a statement.
The professor, who was educated at Stanford in the United States and works from a lab in the southern Chinese city of Shenzhen, said their DNA was modified using CRISPR, a technique which allows scientists to remove and replace a strand with pinpoint precision. "It's just nearly surreal", said Eric Topol, founder and director of the Scripps Research Translational Institute, who said he has seen some of the data behind the experiment.
Dr. Eric Topol, who heads the Scripps Research Translational Institute in California, said, "We're dealing with the operating instructions of a human being".
George Church, from Harvard, however, defended this research. He told the AP that since HIV is "a major and growing public health threat" he finds such experiments "justifiable".
"Not only do I see this as risky, but I am also deeply concerned about the lack of transparency surrounding this trial", said Mr. Zhang, reiterating his support of a moratorium on implantation of edited embryos. He's goal was to introduce a rare, natural genetic variation that makes it more hard for HIV to infect its favorite target, white blood cells.
The gene editing tools modified a gene called CCR5.
Such genetic tinkering contravenes global ethical guidelines and some countries' laws regulating the use of gene-editing in human reproduction - which some call the slippery slope towards designer babies. He has even applied for patents for some of his methods as a result of his study.
Under Canada's Human Reproduction Act, such germ-line editing is illegal and could be punishable by up to 10 years in prison.
Zhang says causing a genetic adjustment may have additional unintended consequences - for example, in this instance, causing increased likelihood of contracting West Nile Virus or dying from the flu.
The Associated Press reported on Monday that He sought and received approval for his project from the ethics committee of Shenzhen Harmonicare Women's and Children's Hospital, and an approval document from the hospital circulated online on Monday.
That type of gene editing is banned in the US except in lab research. First, sperm was "washed" to separate it from semen, the fluid where HIV can lurk. A single sperm cell, he says, was placed into a single egg and an embryo was thus formed. Several days later, the cells of the modified embryos were checked for signs of DNA editing.
Couples chose whether they wanted to try to get pregnant with edited or unedited embryos. Back in 2016, scientists in China used CRISPR to introduce a beneficial mutation that disables an immune-cell gene called CCR5, conferring immunity by knocking out a critical receptor, or mode of entry, for the HIV virus to infect a cell.
Tests suggest the technology edited both copies of the gene in one of the girls, whereas only one copy was changed in her twin. Chances are that it's already happening.
He said that, as there now existed a convenient and practical method to prevent HIV infection, using genome editing for preventive purposes was like "using a cannon to shoot a bird".