An edited essay, featuring several red pen marks.

IN THE summer of 2005 Karen Aiach and her husband received heartbreaking news about their four-month-old daughter, Ornella: she had a rare disorder known as Sanfilippo syndrome. The prognosis was that, from about the age of three, the disorder would gradually rob her of most of her cognitive abilities. She would probably develop a severe sleep disorder and become hyperactive and aggressive. She was unlikely to live into her teens; she certainly would not survive them.

The problem was that Ornella lacked a working copy of a specific gene. It is a gene that tells the body how to make a particular protein which is involved in clearing up cellular debris. Without that protein the cells of her body were unable to break down a complex sugar molecule, heparan sulphate. It is the build-up of that molecule in brain cells that lies behind the symptoms of the syndrome. If her cells could make that protein, the situation might, in principle, be reversed. Learning this, Ms Aiach embarked on a ten-year search for a way to correct the error in her...