Joshua Hartley (back, right) had his life-saving operation last year

Nathan Hartley and his brothers Daniel, Joshua and Luke, from Romsey, Hants, suffer from the genetic disorder X-Linked Lymphoproliferative Syndrome.

Nathan will have his transplant on 29 September at Great Ormond Street.

Joshua, 13, had a transplant in June 2004, Daniel, nine, had his in November, while Luke, five, is scheduled to undergo surgery next year.

The fact that Daniel hasn't been that good until the last couple of months is what has delayed Nathan going in The boys' father, Mr David Hartley

The boys' father, Mr David Hartley, said: "We are okay about it, but we know that there are potential dangers but we have to do it.

"Both Joshua and Daniel are very well at the moment. It's all heading in the right direction.

"The fact that Daniel hasn't been that good until the last couple of months is what has delayed Nathan going in.

"We are all doing pretty well at the moment. We have had a good summer.

"The boys have been as normal as it gets."

World's rarest fatal disorders

The brothers were diagnosed with X-Linked Lymphoproliferative Syndrome, or Duncan's Syndrome, which attacks the immune system, in November 2003.

Their parents, David and Allison, were told their sons were unlikely to reach their teenage years without a bone marrow transplant.

Joshua had his operation after his mother Allison was found to be a match.

The syndrome, is one of the world's rarest disorders, affecting about 100 families world-wide.

Daniel was due return to Romsey Abbey School on 2 September, for the first time since his transplant.

Mr Hartley added: "He can't wait, he's really looking forward to it.

"He has had an excellent home tutor which has made a big difference and has really helped him keep up but he is looking forward to seeing all his mates."