TINY Alex Herbert lies sealed from the outside world as he battles against a rare genetic disorder which claimed the life of his seven-year-old brother only three months ago.

The four-year-old underwent a bone marrow transplant last week, the only cure for XLP, which attacks the immune system and turns the most minor infection into a deadly threat.

So far, he is responding well, but side effects from the treatment and accompanying chemotherapy are taking their toll.

For parents Ian and Alyson Herbert, hope is tempered by the loss of their son Edward, who died in the same Bubble Unit at Newcastle General Hospital.

Mrs Herbert, 36, said: "For Alex, it is so far so good. You never build your hopes up because we have seen too much anything can happen."

Alex has also undergone his first blood transfusion and last night was feeling a little better and his appetite had improved.

Doctors discovered it was XLP which caused aplastic anaemia in Edward which prevents the bone marrow producing the blood cells which fight off infection only two months before he died after undergoing two bone marrow transplants.

The family, who live near Barnard Castle, County Durham, also faced the trauma of knowing that Alex had also been born with the same disorder.

However, eldest son Michael, nine, is free from XLP and it is he who has given Alex his best chance of survival.

Mrs Herbert added: "Michael was a perfect match for a bone marrow donor which is the only bit of good news we have had. As soon as we told him, he offered to be a donor.

"It is difficult coping, it brings it all back being here in the hospital and going through all the same medical procedures.

"That is the most difficult thing for Alex. He is reminded of Edward in hospital, which has made him quite sad, but he likes to be brave, just like Edward was. That is always his goal.

"We are so proud of how bravely Edward fought the illness and how he put up with so much pain and discomfort. He never complained."

Mr Herbert, 37, said: "Alex is having an uncomfortable time but so far he is not experiencing all the complications Edward had. But we are just taking it one day at a time."

Alex is expected to remain in hospital for a further three months.

The family's ordeal began in April last year when Edward caught a virus which affected his bone marrow.

He was placed on antibiotics and was in and out of hospital until he was admitted to Newcastle General Hospital in December 2000 for a bone marrow transplant, donated by his father, which was not a perfect match.

Mr Herbert said: "Within about two to three weeks it failed. All this time he was suffering terrible side effects and had to undergo up to ten other related operations."

Edward underwent a second transplant in February, again using bone marrow donated by his father, along with higher levels of radiotherapy and chemotherapy.

Initially, Edward received almost weekly blood transfusions but during the final months this increased to every other day.

He was transferred to the Royal Victoria Infirmary in March to continue treatment, but two months later returned to Newcastle General after a virus caused fluid on his lungs. He never recovered.

Mr Herbert added: "When Edward was first diagnosed, we thought it was the end of the world but it just got worse. At his funeral we knew Alex was going to have to go through the same course of treatment."

The couple have also had to endure the constant threat of foot-and-mouth to the family farm and a devastating barn fire. Mrs Herbert said: "This last year has been total hell.We have to cope, there is no option."

The couple have been lifted by the support they have received from medical staff as well as family, friends and workmates.

Staff and inmates at Deerbolt Young Offenders Institution at Barnard Castle, where Mrs Herbert works as a librarian, have raised nearly £3,000 for the Bubble Appeal which funds medical equipment, parental support and research for the Bubble Unit at Newcastle General.

The unit, which caters for children from as far afield as Birmingham, Scotland and Ireland, contains six rooms blasted with sterile air to form a protective bubble to protect young patients from infection.

Mr and Mrs Herbert are also appealing for people to become both blood and bone marrow donors. Those giving blood can also opt to be considered for the Bone Marrow Register.

"You think that children don't die.You don't realise how important these things are until you come into a hospital like this," said Mrs Herbert.