The last of four siblings who all have a rare genetic disorder is to have a lifesaving bone marrow transplant - thanks to his brother.
The boys all suffer from one of the world's rarest genetic disorders
The Hartleys from Romsey have X-linked Lymphoproliferative Syndrome (XLP), with sufferers not expected to live into their teens without a transplant.
Luke, six, is due to have a transplant in March, with Nathan, 13, who in 2005 had successful treatment, as a donor.
Only about 100 males have the immune system disorder worldwide.
The boys' father, David Hartley, told BBC News that Nathan would act as a donor to Luke.
He said: "The good folks at Great Ormond Street Hospital [GOSH] learnt last week that the donor we were expecting to use is now no longer available - we know no more than this.
"The consultants at GOSH are not worried by this and have already carried out a number of piggyback bone marrow transplants.
"Nathan is delighted that he will have the chance to save the life of his brother."
David and Allison learnt at Christmas 2003 that their four sons had XLP. The disorder affects only boys, and was discovered in 1999.
After a nationwide appeal, through the Anthony Nolan Trust, donors were found for Daniel, now 10, and Nathan 13, and Mrs Hartley was able to act as donor for their eldest son Joshua, 14.
The boys now have a normal life expectancy.