The BBC's Pallab Ghosh on a breakthrough in cancer research

Previously it was thought that cases of this blood cancer were "one-offs" but scientists from the Cancer Research Campaign now think the disease has an hereditary link and could run in families.

Research, published in the medical journal The Lancet, shows that almost 20% of sufferers of chronic lymphocytic leukaemia (CLL) could have a mutated version of the gene known as ATM.

Of these one in three may have inherited the fault.

Lead researcher Malcolm Taylor said: "We have suspected for some time that a number of patients with this type of leukaemia could be carriers of a faulty gene.

"But the fact that we have been able to isolate the specific gene and show that it could run in families is certainly a major scientific step forward."

Professor Taylor said the discovery should help scientists develop new ways to repair the defective gene and to stop the onset of CLL.

CLL is the most common type of leukaemia with 90% of cases affecting people over the age of 50.

The disease can weaken the immune system and leave sufferers susceptible to other serious infections.

But in most cases CLL is not immediately life-threatening and its progression can be effectively controlled by drugs for many years.

Professor Taylor's team believe, however, that problems with the ATM gene could be responsible for a more aggressive form of the disease.

Blueprint of life

Professor Gordon McVie: "An exciting development"

Genes - which contain the blueprint of life - come in pairs and if people are born with one copy damaged, they are more likely to develop cancer if exposed to other cancer-causing agents.

The body has an in-built "safety-device" which tells damaged cells to "commit suicide" before they have a chance to grow into a potentially fatal tumour. The ATM gene is believed to be involved in this process.

If the gene is "switched off" it cannot instruct cells to die and, therefore, cancerous cells are able to continue to multiply out of control.

The new research showed that in 40% of cases the function of ATM was reduced, and in 15% of cases it was not working at all. This meant that the suicide response could not be triggered in rogue cells.

Professor Taylor's team also found that some patients carried the ATM mutation in all their body cells, not just those in the tumour. This indicates that the mutation has been passed down genetically from their parents.

Professor Taylor said not only would the discovery pave the way for research into repairing the rogue gene, it would also have implications for the way in which some leukaemia sufferers are treated with current chemotherapy and radiotherapy techniques.

It is already known that sufferers of the rare genetic disorder Ataxia Telegiectasia - in which both copies of the ATM gene are inactive - are unusually sensitive to radiation.

This sensitivity could be exploited to treat leukaemia sufferers, or it might mean that standard doses of radiation could be dangerous.

Professor Gordon McVie, director general of the Cancer Research Campaign, said: "These findings are extremely exciting because they are taking us yet another step closer to finding out what causes cancer.