IN JULY 2019, Rohan Seth published an emotional message on Twitter. It said that his six-month-old daughter, Lydia, had been born with a random gen­etic mutation. He had been told that she would live a life full of pain and never sit up, crawl, walk or talk. Doctors said that nothing could be done. But Mr Seth explained that he planned to use a technology called antisense oligonucleotides (ASO) to create a personalised drug just for his daughter.

Every human being has about 6bn letters of DNA in their genome. Only a single letter need go wrong to cause a devastating illness. There are about 10,000 known genetic diseases. Although such genetic errors are rare, collectively they are likely to affect tens of millions of people around the world. The more common genetic diseases, such as cystic fibrosis, haemophilia and sickle-cell anaemia, are receiving attention from pharmaceutical firms. The really rare ones, like Lydia’s, are not.

ASOs work by interfering with the message sent by DNA. If the genetic error is known, it is possible to synthesise a nucleotide molecule that will bind to the message the gene sends. This turns the faulty gene off. The first ASO drug approved by America’s Food and Drug Administration (FDA) was fomivirsen, for cytomegalovirus retinitis, an eye infection, in 1998. Since then the technique has been used in other approved drugs. ASOs can be designed and synthesised swiftly, allowing a treatment to be created in months.

That is exactly what Timothy Yu, at Boston Children’s Hospital, did in 2018 for a girl with Batten’s disease—a rare and fatal neurodegenerative disorder caused by the build-up of proteins and lipids in the brain. Within a year of the child’s diagnosis, he had developed a therapy tailored to her specific genetic mutation. The treatment appears to be helping a great deal.

Pharma firms are required by the FDA to test new medicines on a group of patients over a number of years. But the FDA seems to be willing to allow these unique treatments to go ahead, because there is no other option and the outcomes without them are grim. In medical ­trials, the number of patients is denoted by the letter “n”. These personalised medicines have therefore become known as “n of 1” drugs.

“We don’t have years, we have months,” says Mr Seth. In 2020, on the website savelydia.com, it will be possible to track the family’s progress creating the new drug and giving it to Lydia. The family says it has “realistic” goals. They don’t expect a cure, but believe they can dramatically improve her quality of life.

The number of n of 1 drugs will grow in 2020. At least five other trials are thought to be under way. Mr Seth has an eye on other children beyond his daughter. As part of the work, he has created an open-source platform­­, called the Lydian Accelerator, which will gather and share data on everything needed to create n of 1 drugs. The family is trying to raise $2.5m but hopes that the cost of n of 1 treatments will eventually come down to a few hundred thousand dollars.

If that sounds like a lot, it is worth considering that one of the latest gene-therapy cures for a genetic disorder, Novartis’s Zolgensma, has a list price of $2.1m. Over time, as n of 1 drugs become more common and attract the attention of charities and foundations, an odd situation could arise. Completely personal and on-demand therapies could end up costing far less than something created by the industry for a larger number of patients. These drugs could thus prove disruptive to the industry. Necessity is the mother (and father) of invention.

This article appeared in the Science and technology section of the print edition under the headline "Introducing "n of 1" drugs"