Doctors at London's Great Ormond Street Hospital have carried out a pioneering bone-marrow transplant technique.
They say the method should help with donor shortages since it does not require a perfect cell match.
Mohammed Ahmed, who is nearly five years old, was among the first three children in the world to try out the new treatment.
He has severe combined immunodeficiency syndrome and had been waiting for a suitable donor for years.
"We think Mohammed is cured of his disorder. He should be able to lead a fairly normal life now."
A full report about Mohammed's therapy and the research by Great Ormond Street Hospital, King's College London and the Institute of Child Health has just been published in PLoS One journal.
There are currently about 1,600 people in the UK waiting for a bone-marrow transplant and 37,000 worldwide.
Just 30% of people will find a matching donor from within their families.
Donations involve collecting blood from a vein or aspirating bone marrow from the pelvis using a needle and syringe.
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