In November 2018, a scientist named He Jiankui announced that he had used CRISPR-Cas9 to edit the genomes of human embryos, producing twin girls with a mutation intended to make them resistant to HIV. The announcement was met with near-universal condemnation from the scientific community.
He was subsequently sentenced to three years in a Chinese prison. The twins, named Lulu and Nana, are now seven years old, carrying permanent, heritable genetic changes — the first humans ever to have their germline edited.
What CRISPR actually does
CRISPR-Cas9 is a molecular tool derived from a bacterial immune system. Cas9 is an enzyme that cuts DNA at a specific location; a guide RNA directs it to the correct sequence. By introducing both components into a cell alongside a template for the desired change, it’s possible to delete, correct, or replace specific genetic sequences with remarkable precision.
In principle, this could be used to eliminate heritable genetic diseases. Huntington’s disease, cystic fibrosis, sickle cell anaemia — conditions caused by well-characterised single-gene mutations — would seem like candidates.
The problem with He Jiankui’s experiment
Several things made what He did profoundly unethical, beyond the procedural violations. First, the mutation he introduced — disabling the CCR5 gene — does reduce HIV susceptibility, but the twins were not at elevated risk of HIV. There was no therapeutic justification proportionate to the risk. Second, CRISPR is not perfectly precise: off-target edits, where the enzyme cuts in unintended locations, remain a real concern, especially in early embryos where a single mistaken edit is replicated in every cell of the developing organism. Third, the changes are heritable. Any children Lulu and Nana have will potentially carry these edits. The consequences extend indefinitely into future generations.
The legitimate debate
Separating He’s reckless experiment from the underlying question, there is a genuine ethical debate about germline editing worth engaging with.
Arguments in favour (under rigorous conditions): there is no meaningful moral difference between preventing disease before birth and treating it afterwards; parents routinely make decisions with significant consequences for children who cannot consent; some genetic diseases cause severe suffering and currently have no cure.
Arguments against: the distinction between therapy and enhancement is inherently unstable — if we permit editing to eliminate disease, the path to editing for traits like height or intelligence becomes harder to defend. Germline editing creates irreversible changes in people who have not consented. Access is unlikely to be equitable. And the long-term effects of even well-intentioned edits cannot be predicted.
The UK has one of the more carefully regulated environments for this research — the Human Fertilisation and Embryology Authority permits certain research uses of CRISPR in embryos under strict conditions, but clinical use remains prohibited. The scientific consensus is that basic research can continue, but implantation of edited embryos should await substantially stronger safety data and broader societal agreement on the ethical boundaries.
It is one of the few questions in medicine where the science is moving faster than the ethics, and where the decisions made now will affect people who don’t yet exist.