The last of four brothers diagnosed with a rare fatal illness will go into hospital for life-saving treatment.
Luke Hartley and his brothers Daniel, Nathan and Joshua , from Romsey, Hants, suffer from the genetic disorder XLP.
Joshua, 15, Daniel, 11, and Nathan, 13, have all had bone marrow transplants, only Luke, seven, has not.
He will undergo 10 days of chemotherapy to destroy his bone marrow to make room for a new marrow transplant in June at London's Great Ormond Street Hospital.
Nathan will donate his bone marrow, which will be extracted through a needle injected in his thigh while he is unconscious by doctors at the hospital.
"We have to cross this last hurdle, when Luke goes back to school that day will be amazing"
He will stay in overnight and will feel sore for a few days.
The brothers' father, David Hartley, 44, said: "Nathan realises he is doing a huge thing for his brother so it's a relatively small sacrifice."
The siblings were diagnosed with XLP (X-Linked Lymphoproliferative Syndrome), or Duncan's Syndrome, which attacks the immune system, in November 2003.
Their parents, David and his wife Allison, 40, were told their sons were unlikely to reach their teenage years without a bone marrow transplant.
Joshua had his operation in 2004 after his mother was found to be a match.
Nathan and Daniel found donors after thousands responded to a massive campaign by the Anthony Nolan Trust.
'Absolutely devastating'
XLP is one of the world's rarest fatal disorders affecting about 100 families world-wide.
Mr Hartley added: "XLP is a time bomb and it can go off very quickly and the results can be absolutely devastating.
"Whilst Luke is incredibly well at the moment we know if he doesn't go through this process at some point in the not too distant future we would face a major problem.
"We have to cross this last hurdle. When Luke goes back to school, probably in October or November, that day will be amazing.
"We know we still have a long march along this path but at least we can see the end now."
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