Researchers have pinpointed a gene, known as ATM, thought to be responsible for some cases of the most common type of leukaemia. It is the first evidence of a hereditary link to the disease.
The findings from scientists at Birmingham University, published in The Lancet yesterday, show that almost 20 per cent of sufferers of chronic lymphocytic leukaemia (CLL) could have a mutated version of the gene. Of these, one in three may have inherited the fault.
Professor Malcolm Taylor, the chief researcher, said: "We've 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 a major step forward.
"This means that we can start looking for new ways to repair the defective gene and ultimately prevent this type of blood cancer developing."
CLL, which affects people over 50 in 90 per cent of cases, is not immediately life-threatening and can be effectively controlled by drugs. Its main danger is in weakening patients' immune systems and leaving them open to infection.
However, Prof Taylor and his team think problems with the ATM gene could also be responsible for a more aggressive form of leukaemia.
The ATM gene is thought to be involved in apoptosis, the self-destruct mechanism by which damaged cells commit suicide before they have a chance to grow into a potentially fatal tumour. If the gene is "switched off" it cannot instruct cells to die, so they are able to continue multiplying out of control. The team found that in 40 per cent of tissue samples taken from sufferers of CLL, the ATM gene was either impaired or not working.
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."