Doctors have successfully rewritten a baby's DNA to cure a rare and potentially fatal genetic disorder in what experts are calling an "extraordinary" world first.

The breakthrough could pave the way for more personalised gene-editing therapies in the future.

The infant, known as KJ, was born with carbamoyl phosphate synthetase 1 (CPS1) deficiency, an extremely rare condition that affects only one in 1.3 million people and kills half of those affected in infancy.

Independent experts have hailed the breakthrough as “totally extraordinary” after doctors were able to treat a severe disorder that affects only one in 1.3 million people and kills half of those affected in infancy.

Doctors have successfully rewritten a baby's (not pictured) DNA to cure a rare and potentially fatal genetic disorder in what experts are calling an 'extraordinary' world first

CPS1 deficiency prevents the body from properly processing ammonia, which is created when proteins are broken down in the liver.

Normally, an enzyme converts this ammonia into urea that can be passed in urine.

Without this process functioning correctly, ammonia builds up to toxic levels, potentially causing organ failure and brain swelling.

Babies with this condition typically must wait until they're old enough for a liver transplant, surviving on a low-protein diet in the meantime.

Researchers from the Children's Hospital of Philadelphia, where the baby was cared for, and the University of Pennsylvania used CRISPR gene-editing to make precise edits to DNA within living cells, correcting the specific genetic fault causing KJ's CPS1 deficiency

Many don't survive long enough for transplantation.

Researchers from the Children's Hospital of Philadelphia, where the baby was cared for, and the University of Pennsylvania used CRISPR gene-editing to make precise edits to DNA within living cells, correcting the specific genetic fault causing KJ's CPS1 deficiency.

They delivered the corrected cells to the liver using tiny particles of fat.

This marks the first time gene-editing technology has been used to create a bespoke therapy for a single patient with one specific mutation, rather than addressing multiple mutations behind a disease.

Dr Rebecca Ahrens-Nicklas is a geneticist at CHOP

The baby received a very low dose of the treatment at six months, which was then increased as he grew older.

Dr Rebecca Ahrens-Nicklas, a geneticist at CHOP said: "We knew the method used to deliver the gene-editing machinery to the baby's liver cells allowed us to give the treatment repeatedly, which meant we could start with a low dose that we were sure was safe."

KJ has responded well, eating higher levels of protein and effectively fighting off infections, though he will likely need lifelong monitoring.

Professor Kiran Musunuru from the University of Pennsylvania said: "The promise of gene therapy that we've heard about for decades is coming to fruition, and it's going to utterly transform the way we approach medicine."