AT six years old, Lydia Mellen is a bubbly, happy little girl who loves to ride horses.

But Lydia is also unlike most other children her age - she has a very rare liver disease and now faces a desperate wait for a transplant to survive.

Lydia, of Shirley Avenue, York, is one of a very few children who have a disease called Progressive Familial Intrahepatic Cholestasis (PFIC).

By a cruel twist of fate, she inherited it from her parents Richard and Sam - both of whom happened to be carriers of a particular mutated gene which has serious consequences when combined with another of the same type and passed on to a child.

Neither knew they had the abnormal gene until 2000 when Sam gave birth to Lydia - who was a perfectly healthy baby but became desperately ill at about eight weeks old.

She now faces being put on a transplant list after spots were found on her liver in a recent MRI scan.

Richard said: "Sam and I have latent genes which the majority of the population have but it's very rare that two people with the same mutated gene will meet and have children and cause deficiency of the liver.

"We only know of three children at the moment with PFIC - one in Wakefield and another in Liverpool.

"Her liver is progressively getting worse to the point where it will stop working completely and she will require a transplant.

"If she doesn't get a liver transplant, it's terminal. There are very few children who survive liver disease without a transplant - it's one of the horrible truths about it.

"Ninety-eight per cent of PFIC children are transplanted before they're ten years old. She needs a liver before she's ten and it's become more and more immediate that she's going to need a transplant very, very soon."

Lydia's unusual condition baffled doctors for two years when she first fell ill with jaundice as a baby.

She was taken to St James's Hospital in Leeds, suffering from rapid weight loss and so desperately ill her parents feared she may not live.

After two years of continual monitoring and tests, when Lydia had to be fed through a special pump, a German professor finally diagnosed her condition.

Richard and Sam have been told by medics that if they decided to have another child, there would be a one-in-four chance of them having the same disease.

Today, Lydia appears to be a normal little girl who goes to Carr Infant School and loves horses. The only outward sign of her illness is her tiny stature - a symptom of the disease.

But every morning and night, she has to be given six different types of medication, fed to her through a permanent tube in her stomach.

If she undergoes a liver transplant, she will be on medication for the rest of her life.

Richard, who runs Savilles Cook Shop in Malton, said: "She's tiny - she's about the size of a four-year-old. She's obviously not thriving like a normal child physically but academically, she's very, very smart. She's well looked after at school.

"She's brilliant. She's very bubbly, very happy, very cheeky.

"Liver disease affects everybody. It's not just for people who drink themselves to death. Liver disease does affect small children and it's devastating when it does.

"It's taken us six years to come to terms with it and it's hard every day. It's just a horrible, horrible disease."

But there is new hope for Lydia, after the first live liver transplant on the NHS was performed at St James' Hospital last month.

The Leeds doctors made history on June 21 when they carried out the eight-hour operation on Stephen Lomas, from Cumbria, who had been given part of his son David's liver.

Medics have hailed the ground-breaking operation as a new hope for other patients on the liver transplant list.

Previously, those on the list had to wait for a donor to die before being given a transplant - and one in five died waiting.

Richard said the operation had given the Mellen family hope that Lydia too might benefit from a live liver transplant - as opposed to having to wait for a dead donor.

He said: "One can only hope that when the inevitable happens, we won't have such a long wait to find a suitable donor for our daughter.

"Perhaps in the future, a donor register similar to the Anthony Nolan Trust could be set up for people to offer partial lobes of their livers to those on the waiting list. It could go someway to helping with the shortage of organ donors in the UK and in Europe.

"We're hoping there's going to be match on the maternal side of the family - from my wife or sister-in-law.

"Although the NHS comes under attack from all angles, St James' is one of the pioneers of treatment.

"Yes, the NHS is under-funded but our daughter has had nothing, but the best since she was born."