According to a paper detailing the findings of the experiments, published in the Journal of Assisted Reproduction and Genetics, the technique was performed on 45 fertilized, non-viable human eggs. The scientists were able to introduce a naturally occurring genetic mutation named CCR5 Δ32 into the embryos. This mutation, when present in humans, makes the T-cells, which are a subtype of white blood cells, immune to the HIV virus.
Of the 45 embryos injected with the RNA instructions during their one-cell stage, only 26 developed into eight-cell embryos and beyond. And, out of these, only four acquired the desired mutation. Editing human embryos is problematic because it could have long-term, unintended consequences. One such effect, wherein a surprising number of “off-target” mutations that act on other parts of the genome come into being, was visible in this particular study as well as the previous one carried out last year.