Thirteen of 16 children who received a bone marrow transplant using a new antibody treatment rather than chemotherapy have survived. Three of the families describe the experience.
Kasey was born with a serious immune deficiency which affected his gut. Fostered from the age of four months old, little was known about his genetic history, until tests diagnosed him at age four.
Kasey has just learned to ride a bike
He had to be fed intravenously, and in the first five years of his life was in and out of hospital. He was too ill to receive the conventional chemotherapy-based transplant, so doctors recommended the antibody treatment.
His foster mother Brigitte Cadd says she was told few children with the condition live beyond the first year.
"At this stage we had already lost him a couple of times, so I was in no doubt about whether we should go ahead with this new treatment," she said.
"It was very soon after the transplant we could tell it had worked beautifully."
Kasey, from East London, can now eat normally although he is still fed once a day through a gastric tube to reduce the strain on his gut. Although he is still small for his age, he was able to start school at seven and is said to be making good progress.
And the boy who once had "no motor skills" has just learned to ride a bike.
Ffion was only three weeks old when she developed skin problems so serious she was frequently in hospital. Unable to sleep for more than an hour at a time because of the discomfort, her skin could turn bright red and weep. At three months, Great Ormond Street doctors diagnosed her with Omenn Syndrome, a severe immunodeficiency.
Her mother Louise Moseley, from Pemborke Dock, Pembrokeshire, said: "All I heard was 'she needs a bone marrow transplant. It's a fatal condition'.
Ffion was diagnosed with Omenn Syndrome at three months
"It was a complete shock. I've never heard of it because it's such a rare condition."
The transplant was carried out when Ffion was five months old. She did suffer complications in the aftermath, including Graft Versus Host Disease - where the body attacks the new tissue - but was well enough to go home three months later.
Her skin, says her mother, is now "beautiful".
"Now she's a completely healthy, happy little girl who goes to school, she's outgoing and bubbly, and you'd never know she's been through all this.
"I think it's really important to remember what happened and how lucky we are to have her."
Jamie was born with a condition so severe he effectively had no immune system. His mother Rebecca Reece said he was ill "all the time" with chest infections, coughs and temperatures.
After a night when he was unable to stop coughing, his parents took him to the paediatric intensive care unit at Addenbrooke's in Cambridge, where a diagnosis was made.
Jamie's parents now refer to him as the "stuntman"
Like the other children in the trial, the little boy was so sick that he was unlikely to have survived the conventional treatment.
When offered the procedure using antibodies, "we just said yes, yes," said Mrs Reece.
Despite some complications following the operation, within a few months Jamie had made an amazing recovery. He now attends hospital once every six months.
"We call him our little stuntman. He runs everywhere. He just doesn't sit still."