Mark Lee knows how devastating living with haemophilia can be.

Key points: Gene therapy trial stops bleeding in patients with haemophilia B

Gene therapy trial stops bleeding in patients with haemophilia B RPA scientists call it "a major milestone"

RPA scientists call it "a major milestone" Patient Mark Lee says it's "life-changing"

The genetic illness, where the blood does not clot properly, can lead to bleeding in the joints and muscles.

He has the disease and lost his two brothers to complications from haemophilia B when they were young.

But he has not had any bleeding since taking part in a ground-breaking Australian clinical trial.

Professor John Rasko lead a groundbreaking clinical trial attempting to cure haemophilia B. ( LinkedIn )

Ten adults with haemophilia B were injected with a modified gene, designed to produce a substance which clots the blood, called Factor IX.

Lead researcher Professor John Rasko, from the Royal Prince Alfred Hospital, found after following the patients for a year, nine out of the 10 did not have any bleeding.

"This is a major milestone in the quest to cure the bleeding disorder," he said.

Mr Lee, a 33-year-old from NSW, said being in the trial was "life-changing".

"I spent my childhood wrapped in cotton wool, unable to play football or do any of the things my mates could," he said.

How did they do it? The researchers modified a gene which creates Factor IX

The researchers modified a gene which creates Factor IX The gene was encased in a harmless virus then injected into the patients

The gene was encased in a harmless virus then injected into the patients The virus casing allowed the gene to infiltrate the liver cells, telling them the make Factor IX

Professor Rasko said they were very excited about the results, which have been published in the New England Journal of Medicine.

"Those people in the trial have previously had to live with the risks of spontaneous bleeding every day," he said.

The trial was in patients with haemophilia B, which affects about 500 men in Australia.

Researchers will trial the approach in haemophilia A — the most common form of the illness — which affects more than 2,300 Australians.

Professor Rasko believed the results would mean, "the beginning of the end of this life-long bleeding disorder".

"We now know how to beat the immune response to achieve what might be a permanent cure," he said.

Molecular biologist Professor Merlin Crossly from the University of New South Wales said it was a great achievement.

"The idea of curing a human disease like this would have been unthinkable even 50 years ago," he said.

Professor Crossley said the technology had the potential to treat other genetic disorders, but that would take many years.

Mr Lee also has two daughters who are carriers for haemophilia B.

"I know now if they have affected children, it will be one injection and they can live normal lives," he said.

RPA's Professor John Rasko with patient Mark Lee and his 18-month-old daughter Violet and Partner Shannon. ( Supplied )

Approach safe and effective: study

Researchers found the gene therapy technique was safe, with no serious side-effects. Two patients had an increase in liver enzyme levels, which was controlled with medication.

Haemophilia is an inherited condition and men with haemophilia pass the gene onto their daughters.

Women with the gene can pass the gene on to their sons and daughters. Haemophilia Foundation Australia vice-president Daniel Credazzi has a son with the illness.

"The potential of a cure with safe and effective gene therapy is very exciting for people living with this chronic condition," he said.

Gene therapy has been tried before for people with haemophilia, with limited success.

An earlier trial on seven patients in 2006 tested gene therapy as a treatment for the bleeding disorder. The treatment reduced bleeding for 10 weeks in the patients.