A serotonin supplement was added to their medications, and within a month their mother says there were noticeable improvements in Alexis Beery's breathing and in her twin brother Noah's school work.

It seems almost every week there's a story about scientists who have identified a gene linked to some disease or another. Those discoveries are important, but Bainbridge says the Beery case represents something else.

"The difference is that we can actually do something, because we found these mutations and we knew that they were causing the disease. We could actually change the therapy the children got," Bainbridge says.

Whole-genome sequencing is still an expensive, specialized activity. But the cost is coming down, and Bainbridge says this individualized approach to diagnosis and treatment may move out of the research lab and into the doctor's office, at least for some conditions, in as little as two to three years.

"I think eventually it will be routine. I can imagine a future - and not too distantly, in five to ten years - where children will be born and their whole genome will be sequenced immediately."

Bainbridge and his colleagues describe their work in the journal Science Translational Medicine.