A nine-month-old baby born in the United States, KJ Muldoon, has become the first person in the world to be treated with personalised gene-editing treatment in a major medical breakthrough.

According to the New York Times, one week after the baby was born, doctors at the Children’s Hospital of Philadelphia discovered that he had CPSI deficiency. This rare genetic disorder affects one in 1.3 million babies.

Given that half of babies with the disorder die in the first week of life and the survivors eventually experience mental and developmental delays and need liver transplants later on, doctors turned to personalised gene-editing treatment.

The treatment is a result of decades-long U.S. government-funded research, offering a new path for companies to develop personalised treatments without going through years of expensive development and testing.

Genetic disorders, including CPSI deficiency that affected the baby, are mostly the result of a single mutation — an incorrect DNA letter among the three billion in the human genome. To correct this, doctors need to pinpoint targets in an approach called base editing.

To achieve the historic milestone in this case, The Times reported that the treatment is wrapped in fatty lipid molecules to protect it from degradation in the blood on its way to the liver, where the edit will be made.

Furthermore, inside the lipids are instructions that command the cells to produce an enzyme that edits the gene. They also carry a molecular GPS — CRISPR — which was altered to crawl along a person’s DNA until it finds the exact DNA letter that needs to be changed.

The same treatment can be customised repeatedly for other individuals to fix mutations in other places on a person’s DNA.