Just a small fraction of our 20,000 genes can cause disease when disrupted – yet that sliver accounts for thousands of rare disorders. The difficulty is: what can a doctor do to treat them? In a common condition such as type 2 diabetes, the underlying biology is similar for millions of patients. The doctor can prescribe metformin. But with a genetic disorder, the mutation might only affect a small number of people worldwide. In many cases, doctors won’t even know which mutation is responsible, let alone how to fix it.

Novel gene-editing breakthroughs are making headlines. But therapies are expensive and complex to develop. The cost of bringing any new drug to patients is now........

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