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Few recent biological technologies have generated as much excitement—and ethical unease—as CRISPR gene editing. CRISPR systems evolved in bacteria as part of a defence against viruses. Bacteria can retain fragments of viral genetic material and use RNA guides to recognise matching sequences during later attacks. Researchers realised that components of this system could be adapted into a programmable tool. In a widely used form, a guide RNA directs the Cas9 enzyme to a chosen DNA sequence, where Cas9 cuts the molecule. The cell's own repair processes can then disrupt a gene or, under some conditions, help introduce a desired change. The technology is comparatively inexpensive, flexible and easier to programme than many earlier gene-editing methods. Potential applications range from laboratory research and crop development to treatment of genetic disease. Gene-edited therapies have already entered clinical medicine, including treatments targeting sickle cell disease. Yet the ethical landscape differs sharply depending on what is edited. Altering cells in an individual patient affects that person; editing embryos or reproductive cells could create heritable changes passed to future generations. In 2018, a Chinese scientist announced the birth of children whose embryos he had edited, provoking international condemnation over inadequate medical justification, consent and risk. CRISPR illustrates a recurring dilemma in science: technical capability can advance faster than social agreement about when, why and under whose authority that capability should be used.
