Doctors in the US have become the first to treat a baby with a customised gene-editing therapy after diagnosing the child with a severe genetic disorder that kills about half of those affected in early infancy.

KJ was born with severe CPS1 deficiency, a condition that affects only one in 1.3 million people. Those affected lack a liver enzyme that converts ammonia, from the natural breakdown of proteins in the body, into urea so it can be excreted in urine. This causes a build-up of ammonia that can damage the liver and other organs, such as the brain.

Writing in the New England Journal of Medicine, the doctors described the painstaking process of identifying the specific mutations behind KJ’s disorder, designing a gene-editing therapy to correct them, and testing the treatment and fatty nanoparticles needed to carry it into the liver. The therapy uses a powerful procedure called base editing which can rewrite the DNA code one letter at a time.