We're all praying Aiden's brother or sister can save his life.

Baby Aiden Brown is a picture of contentment, a smiling wee smasher ... just like his big brother Aaron, who is nearly three and five- year- old sister, Ashleigh.

But behind all their smiles is an incredible, though tragic, story ...

For 11-month-old Aiden has a very rare illness, Hurler Disease, which will end his life in just a few years - unless he gets a successful bone marrow transplant.

And in a few days the family, who live in Livingston, West Lothian, will know whether or not little Aaron or Ashleigh will be a good enough match to give their baby brother the gift of life.

Both have been tested at a Manchester hospital, where the transplant will take place and now they all pray that Aiden will get his chance to live.

If either Aaron or Ashleigh matches, they will become one of the youngest children in the country to go through the bone marrow donor procedure, although the blood of new- born babies is being used in a pioneering technique in a Bristol hospital.

"Our first hope of saving Aiden lies with Aaron and Ashleigh," said mum Angela, 27.

"We have told them Aiden might need some special blood and that theirs could help him get better. But they don't really understand the truth of our heartache, how serious this is."

She added: "I felt guilty about having my two other children tested, but we are assured that the donor procedure is simple and painless and that they will not be harmed."

Bone marrow will be sucked from either Ashleigh or Aaron's hip, performed under anaesthetic, leaving them with a bruise. The family awaits a 'phone call from the Royal Manchester Children's Hospital at Pendlebury, one of only two hospitals in Britain - the other is Great Ormond Street in London - where treatment for Aiden's condition is carried out by a specialist paediatric team in the genetics unit.

If they get good news, the whole family will stay down in Manchester for six weeks while wee Aiden undergoes his life-saving transplant.

He will have chemotherapy which will destroy his immune system and will have to stay in a sterile bubble for six weeks after the transplant, shielding him from harmful germs.

Just before Christmas, Angela and husband Jonathan, 28, a fire-fighter at Edinburgh Airport, were given the devastating news that wee Aiden had the condition - a very rare, inherited metabolic disease which has affected less than 100 children born in Britain in the last 10 years. Aiden's life-expectancy is five to 10 years as the disease gradually destroys his brain and severely deforms his bones and causes many other complications.

Angela said: "We will go from having quite a normal wee boy to a very severely mentally and physically disabled child."

HIS only hope of avoiding brain damage is the bone marrow transplant ... and in a race against time, he must have the transplant before he is 18 months old, or it will be too late.

If Ashleigh and Aaron are not a good match, it will take another three months to search the bone marrow donor register through the British Bone Marrow Donor Appeal to find one ... and, failing that, just another few months to pray for a miracle.

Angela said: "Even the neighbours in our street have said that if the older children don't match Aiden, then they will all go to be tested. It is a remarkable gesture of support. They all know how vital it will be to find a donor very soon for our baby son."

At the moment, Aiden looks perfectly normal, happy and healthy ... apart from two lumps which have formed already on his chest bones.

His brain development is almost normal. He is already learning basic baby skills.

"It is agonising to know that if he does not get a bone marrow transplant then he will gradually lose all the skills we have taught him," said Angela.

Dad Jonathan added: "We have been absolutely devastated on hearing the news of Aiden's condition.

"It is a nightmare from which we have not yet wakened.

"We had never heard of Hurler Disease. But it turns out from the tests he has had done that he has the most severe form of it."

Angela added: "It is the very worst metabolic disease a child can get. We have learned what it does to children and it is terrible.

"This has shattered all our lives. We are still numb from the shock of hearing what he has. It is so hard to accept. He was an 8lb 5oz baby when he was born on February 23 last year and there was nothing to warn us of what was to hit us in the future.

"We love Aiden dearly. He is such a good baby, always smiling. It tears us apart to know that in a few months his condition will start to deteriorate without his transplant."

Angela and Jonathan are a couple in a million ... it was sheer fluke that they met, fell in love and married, not knowing that they both carried the flawed gene.

Angela said: "We have been told that there is a one in four chance of any children we have being affected.

"It is very fortunate that neither Aaron nor Ashleigh were born with the disease ... because that means that Aiden has a better chance of life, twice as many chances as some children with Hurler who are the first-born.

"I knew from almost the day Aiden was born that there was something wrong with him, but paediatricians kept telling me he was OK.

"But when they finally did tests, never in a million years did we dream that it would be something so awful as Hurler Disease. We try to take each day as it comes. But we know we don't have time on our side."

THE Browns are now in touch with the Society for Mucopolysaccharide Diseases, which gives them vital support so that they do not feel completely isolated with their trauma.

A spokesman for the national charity said: "Discovering that your child has an MPS or related disease is one of the most devastating experiences a family will live through.

"Children with an MPS disorder suffer progressive physical disability. Many lose skills through mental deterioration ... the further tragedy is that the majority of children will die in childhood."

Vital research is being carried out with funding from the charity, including gene therapy (replacing the faulty gene with a copy of the normal one), but the main hope for little Aiden now is his bone marrow transplant.

"Aiden will still have some problems with disability in the future," said Angela.

"But it is our dearest wish that we can see him reach his teens."