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Paddy Doherty leaned back in the narrow hospital bed and watched a bag of clear liquid drain into a vein in his left arm, whisking billions of invisible strands of mRNA into the torrent of his bloodstream.

It was only in February he’d learned the pinching pains in his toes and gasping for breath that had begun to haunt his daily dog walks through the hills of Donegal County were caused by a deformed protein piling up in his nerves and heart. Now here he was, a few months later, at a clinic in central London, so that doctors could try to fix the genetic defect he’d inherited — the same one that had already killed his father, an uncle, and a cousin.


If all went according to plan, over the coming days these mRNA molecules would deliver a message to Doherty’s liver, directing the cells there to make something they’d never made before: the DNA-editing machinery known as CRISPR. Once constructed, these new CRISPR molecules would make their way to chromosome 18, latch onto his glitchy TTR gene and snip it in two, preventing the liver from pumping out any more of that deformed protein. If it worked, Doherty might be cured of the rare but deadly disease transthyretin amyloidosis that had run through his family for generations.

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