The four-year-old identical twins who could help find a kinder cure for cancer

But one day identical twins Olivia and Isabella Murphy may be told of their unique contribution to the battle against cancer.

Olivia, a bubbly, pretty four-year-old, was diagnosed with leukaemia two years ago and has just completed a successful, but painful, course of chemotherapy.

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Despite sharing exactly the same genetic make up and upbringing, her sister Isabella has been healthy since birth.

An unprecedented medical study on the girls by Leukaemia Research has now enabled doctors for the first time to prove the existence of leukaemia stem cells - rare and deadly mutants in bone marrow which can develop into any of the different types of cancerous cell.

They hope the discovery will assist the development of more intelligent treatments which can seek out and destroy such cells before they progress.

When Olivia first fell ill in June 2005 doctors thought her fever and sickness were the result of tonsillitis.

But after her parents Sarah and Justin insisted on a blood test, they found she had acute lymphoblastic leukaemia.

Treatment with three month-long courses of chemotherapy was a success.

However, her immune system was so badly hit she suffered six attacks of shingles which left her blind in one eye.

During the treatment, her parents agreed to let researchers take regular blood samples from both girls. The tests allowed them both to screen Isabella for early signs of the disease and to explore its causes.

All cancers are caused by genetic mutations which cause cells to divide and multiply uncontrollably.

Leukaemia is caused by two different mutations - one which occurs in the womb, the other after birth.

Professor Tariq Enver, who led the study at the MRC Molecular Haematology Unit in Oxford reported in the journal Science, said both girls had the first mutation, but only Olivia had the second.

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In Isabella, the 'pre-leuakaemic' stem cells had remained dormant. But Olivia's had turned into full-blown cancer stem cells.

"Less than one per cent of the total cancer is the cancer stem cells which can drive and maintain the disease," said Professor Enver.

"Once you know about them this offers opportunities to monitor the disease and develop smarter drugs which kill cancer stem cells."

He believes that it could one day be possible to prevent leukaemia by wiping out the pre-leukaemia cells almost at source. Around 90 per cent of children with leukaemia can be cured.

However, the huge doses of chemotherapy required mean that one to two per cent die from the treatment.

Nine out of ten children who have an identical twin with leukaemia do not develop the disease.

The twins' mother, Sarah, 35, who lives in Bromley, South East London, said the family had been "very happy" to help with the research.

"Anything which leads to a lessening of the intensity of the chemotherapy which can help other families is a good thing," she said.