Scientists in China have reported genetically modifying human embryos in what is only the second published experiment of its kind. Last year, a different team of Chinese researchers edited human embryos in an attempt to remove genes responsible for a dangerous blood disorder. In this new research, scientists from Guangzhou Medical University tried to add a mutation to embryos instead, attempting to make them HIV-resistant. In both cases, the experiments were only partially successful, and were carried out using non-viable human embryos that were incapable of growing into adults.

In this latest research published in the Journal of Assisted Reproduction and Genetics, the scientists collected 213 fertilized human eggs from 87 patients in a fertility clinic. The eggs were all unsuitable for in vitro fertilization because they contained an extra set of chromosomes, and had been donated for research purposes. Using the gene-editing tool CRISPR, the scientists were able to introduce a naturally-occurring genetic mutation into the embryos. This mutation, which modifies an immune-cell gene called CCR5, makes humans that carry it resistant to the HIV virus.

Only four of 26 embryos were successfully modified

The researchers were only able to successfully modify four of the 26 embryos targeted, and some of these embryos also acquired unplanned mutations — a side effect that was similarly observed in the research published last year. After the experiments, the embryos were all destroyed within three days' time.

"The results are both comforting and disturbing," said Dr. Peter Donovan, a professor of biological chemistry and development cell biology at the University of California. "The good news is that the technique worked for this group in the same way that it did for the first group. This indicates the reproducibility of the science [...] However, this group of researchers also reproduced another finding described by the first group, namely that this type of gene editing also causes off-target effects."

These consistent failures have led some scientists to suggest that CRISPR gene-editing techniques are not yet ready to be used in human embryos. Others say that there is still much productive work that can be done, as long as researchers do not cross the clear line of implanting viable genetically-modified embryos into a woman's uterus. At last year's International Summit on Human Gene Editing, researchers from the US, UK, and China agreed that using viable human embryos in research should not be banned, but that altering the DNA of embryos for clinical purposes was unacceptable.

Verge video: An introduction to the gene-editing tool CRISPR

Experiments that genetically modify embryos with the the potential to grow into adults are going ahead. Earlier this year, UK regulators gave the green light to British scientists to conduct experiments on viable embryos. The researchers from the Francis Crick Institute in London will use CRISPR to try and edit out genes that hamper the healthy development of fetuses. The embryos will only be studied during the first seven days following conception (during which time they will grow from one cell to around 250 cells in size), and will be destroyed after 14 days.

Human gene editing isn't ready for the prime time, but research is going forward

"Studies in mice and non-human primates will undoubtedly be informative but ultimately it will be studies like the ones just published using donated human embryos that will give us the most understanding," said Donovan, adding: "There is still much to be learned about gene editing in human embryos before it is ready for prime time."