Stem cells as they divide

But Matthew was too ill to wait a year until his sister was old enough to act as bone donor.

Instead, his parents agreed to take part in a pioneering experiment to use stem cells from his sister's umbilical cord.

After Alison's birth in February 1998, Matthew's diseased blood stem cells were eradicated with chemotherapy, then his sister's cord blood was infused straight into his blood.

Since this successful operation, thousands of children around the world have been helped by the procedure.

Nineteen such operations have taken place in the UK on children with leukaemia. Just over half have been successful.

Much of the work has centred at Duke University Medical Center, in North Carolina, USA.

Cord blood hope

Stem cells are master cells that have the ability to develop into any of the body's tissue types, such as blood, liver, muscle and other cells.

In the past, bone marrow transplants were the only source of stem cells for seriously ill children with blood disorders.

Half a cup-full of cord blood might provide us with enough stem cells to save a child

Professor Hal Broxmeyer, Indiana University School of Medicine

Patients who undergo bone marrow transplants have to be a genetic match with the donor on a 12-point scale - a so-called "perfect match".

Any bone marrow which is not perfect runs the risk of being rejected by the recipient's system - which can be fatal.

The world-wide bone marrow register has 7m names on it, but a third of cases cannot be helped by it.

But they could be helped by blood from umbilical cord, so-called "cord blood".

Dr Joanne Kurtzberg, the doctor who has led the cord blood transplant programme at Duke University Hospital, said: "Cord blood might not just be an alternative [to bone marrow], it might even be better because cord blood doesn't need a prefect match."

She estimated it could help in 90% of cases, where bone marrow could not help.

The attraction of cord blood, say doctors, is that the match does not have to be so close - meaning many more children can be treated.

However, children are still at risk after the core blood transplant.

Because they are waiting for the implanted stem cells to ingraft, or "take", they are left without an effective immune system, leaving them open to infection.

The treatment was first developed by Professor Hal Broxmeyer at Indiana University School of Medicine.

Tests found despite a much smaller quantity of cord blood compared to the litre of bone marrow that was usually transplanted, there were more stem cells in the cord blood.

"That half a cup-full of cord blood might provide us with enough stem cells to save a child," he said.

Success

Lidel Pressey is one of those who was offered a last chance by a core blood transplant.

Lidel Pressey, treated with a cord blood transplant

The little boy had been unable to find a donor because he was of mixed race, and had an unusual genetic make-up.

Two weeks after his transplant, the stem cells had ingrafted, and just weeks after his operation, he was able to go home.

Professor Broxmeyer is now leading work into using cord blood to treat adults with blood diseases.

But the amount of stem cells in the cord blood is insufficient to treat adults, so his team is trying to use growth factors to create enough cord blood.

Initial trials have proved successful in two out of 20 patients treated.

Horizon will be transmitted on BBC2 on Thursday 11 October at 21.00 BST.