Determined that their children have the best possible chance in life, the couple are prepared to undertake further trials if this one fails to produce the cure to the disease, which leads to a progressive deterioration of the brain and nervous system.

The life expectancy of the twins is short, with death normally occurring in mid childhood, but their parents are hopeful trials can provide an eventual cure.

Infantile Battens occurs between the ages of six months and two years of age and affects only one in 30,000 births.

The twins developed the disease as two copies of the defective gene were passed on to them from each parent.

“When both parents carry one defective gene each of their children faces a one-in-four chance of developing battens disease,” explained Alana.

“We would love to have another baby and there are tests you can have to confirm if the baby would have the disease or be a carrier.

“If the baby is a carrier, then it wouldn’t have the disease, but we are still very scared it will happen again.

“The defective gene has probably been in each of our families for generations, but has never affected anyone before because only one parent has it.

“But when two people come together, like Alana and I, that is when, if the defective genes are passed on to the children, they will develop Battens. We did not know we had these genes until the children were diagnosed,” said Mark.

Alana and Mark always thought there was something wrong with Saul and, at nine months old, doctors initially thought he had cerebral palsy because of the tightness of his muscle tone.

However, it was Savannah who showed more obvious symptoms of the disease as her parents noticed that she was often falling over.

Doctors at Ninewells Hospital first thought it could be meningitis. After various tests and scans, it was discovered Savannah-Rose’s brain was quite small and delayed for her age and blood samples were sent to London for tests.

The results confirmed the worst for her parents, who then had the same tests carried out on Saul, which delivered a double blow.

The couple looked into the disease on the Internet and found an American hospital in Bethesda, Maryland, which is doing clinical trials to find a form of treatment.

“To join the trial, children had to be under three and had to be eating orally, which they both were, so they were accepted.

“We returned from our first trip to America four weeks ago, after they underwent similar scans and tests as they had here, but more advanced.

“They have been given a drug called Cystagon and we take them back in six months for the doctors to monitor how the drug has taken affect.

“We were warned the condition may worsen, but we felt it was a risk worth taking if there is a chance a cure could be found.”

The twins are the only children in Dundee with the disease and there are only seven other cases in Scotland.

The couple have set up a trust fund for the children, which today received an initial boost of over £1000 raised from a karaoke night in the Ferry Inn in Broughty Ferry.

Friends and relatives have a number of events set up to help the tots, including a bingo night and sponsored cycles.

The fund will help with travelling costs for any future trials or treatment and to help research into the condition.

“At present, there is no cure, but this clinical trial gives us some sort of hope.

“Although this trial in America is funded, if it doesn’t work out we are prepared to travel elsewhere to take part in further trials.

“That is why we have set up the trust fund and to help research into Battens disease,” said Alana.