LEIGH SALES, PRESENTER: Imagine having to go to hospital for a blood transfusion every month of your life. That's the case for the person you're about to meet.
Stacey Wong has thalassemia, an inherited blood disease that can be fatal.
Now, Australian doctors are using genetic therapy in the hope of offering patients like Stacey a cure.
The ABC's medical reporter, Sophie Scott, has this exclusive report.
SOPHIE SCOTT, REPORTER: Stacey Wong has spent much of her life in hospitals.
Now 26, Stacey's had to have blood transfusions every month of her life just to stay alive, but today could be her last session on the machine.
JOY HO, ROYAL PRINCE ALFRED HOSPITAL: The fact that someone who has an illness that they've been told they will have the rest of their lives can have the prospect of being cured from that illness is a tremendous prospect for them.
SOPHIE SCOTT: Stacey suffers from thalassemia. It's an inherited blood disorder where the body produces fewer red blood cells and less haemoglobin than normal - that's the substance in red blood cells that allows them to carry oxygen. It makes her feel tired and short of breath. Untreated, she could die, but Stacey's been given the chance to live a normal life.
She's part of a groundbreaking genetic trial. At the helm is one of Australia's leading genetic scientists, Professor John Rasko.
Gene therapy changes the way patients are treated by correcting the genetic defect that causes their disease, not just treating the symptoms.
This is how it works. Stacey's stem cells are removed. Scientists in the United States insert a healthy copy of the malfunctioning gene into the stem cells. She's given chemotherapy to wipe out her immune system. Then the genetically-modified cells are transplanted back.
It shows a mishmash of abnormal blood cells which are destroyed prematurely instead of normal red blood cells which last four months.
JOHN RASKO, HEAD, ROYAL PRINCE ALFRED HOSPITAL: If you take not a look at a normal slide that I've got prepared, look at how bland...
SOPHIE SCOTT: Much more uniform.
JOHN RASKO: Much more uniform. They all look the same.
SOPHIE SCOTT: So, could Stacey's cells ever look like this that we're seeing here?
JOHN RASKO: Well that's our hope. One day it's possible to imagine that she'll be manufacturing cells in her own body because we've in a sense reset the balance between the haemoglobin production.
SOPHIE SCOTT: Stacey and her fiancé Christian travelled all the way to this medical centre in Chicago for the first part of the procedure.
Her stem cells were harvested from her bone marrow then genetically modified in a lab using a harmless virus.
One month after her trip to the US, the day of the transplant has finally arrived.
SOPHIE SCOTT: Here in the bowels of Sydney's Royal Prince Alfred Hospital, Stacey's new genetically-modified cells have returned from the US. They're taken out of dry ice and thawed.
It's a big moment. Nothing like this has ever been attempted in Australia before. So much could go wrong.
But Stacey's staying optimistic. She's endured chemotherapy to wipe out her bone marrow and is primed to go.
STACEY WONG, THALASSAEMIA PATIENT: Everyone I know is, like, excited and nervous and I'm just like - they're always like, "Are you worried?" I'm like, "Not really. Why would I be worried?"
SOPHIE SCOTT: Drip by drip the genetically-modified cells are slowly infused.
Then, after years of planning, it was over.
JOHN RASKO: Stacey is such a buoyant individual. She's always positive and she's such a joy to look after.
SOPHIE SCOTT: Eight thalassemia patients around the world including Stacey have had the genetically-modified cells infused. Four have been able to reduce or stop blood transfusions completely and it's looking good for Stacey.
JOHN RASKO: We're already seeing the earliest stages of recovery of Stacey's blood. In particular, the white cells are starting to recover and that indicates to us that the bone marrow's starting to work.
SOPHIE SCOTT: Although she's lost her hair due to the chemo, that seems to be the only downside.
STACEY WONG: Now I feel good and going back to work soon, so, yeah, kind of happy - happy it's all over, but just better than I thought I would be.
SOPHIE SCOTT: And she's been able to go for more than a month without a transfusion.
STACEY WONG: But, ever since the treatment, even though my haemoglobin has gone down a little down, like, the blood count, I don't feel that tiredness and that kind of weakness in my muscles.
CHRISTIAN, FIANCE: Now she's got way more energy compared to before she's had this treatment. All those bits and pieces that she couldn't do before, whether it's going out, staying out longer, she can do now.
SOPHIE SCOTT: And they can finally plan the long overseas honeymoon they've dreamed of.
For Stacey, the risks have been worth the reward.
STACEY WONG: I'm glad I said yes and I feel kind of good that I did it.
JOHN RASKO: This is just the beginning of what we hope is going to be a revolution, not just in thalassemia or just blood diseases, but in many different forms of genetic abnormality. And then, hopefully the sky is the limit.
LEIGH SALES: Sophie Scott reporting.