They have linked mutations in a gene called GATA1 with acute megakaryoblastic leukaemia (AMKL).

This type of leukaemia mostly affects children with Down's syndrome.

This study for the first time defines a part of the molecular pathway leading to acute megakaryoblastic leukaemia

Dr John Crispino

Children with Down's are up to 20 times more likely to develop leukaemia compared to those without the condition.

Dr John Crispino and colleagues at the Ben May Institute for Cancer Research at the University of Chicago hope their discovery will help to pave the way for new treatments for AMKL.

But they also believe it will help other scientists who are trying to understand leukaemia as whole.

DNA tests

They made their discovery after comparing the DNA of 75 patients with various types of myeloid leukaemia and 21 healthy people.

They found that those with AMKL and Down's had the gene defect. None of the others did.

But the scientists warned that further research is needed. They suggested that a number of other genes also play a role in causing the disease.

Dr Crispino said: "GATA1 is just part of the story but it is a crucial early step that should lead us to the rest of the pathway."

But he added: "This study for the first time defines a part of the molecular pathway leading to acute megakaryoblastic leukaemia."

Michelle Le Beau, professor of medicine at the University of Chicago, said the findings would help scientists to improve their understanding of the leukaemia.

She said: "This is a rare malignancy but a great deal of what we now know about the molecular basis of cancer has come from disorders like this.

"Our finding pinpoints a specific pathway that leads to this kind of cancer, offers a method for rapid and precise diagnosis and suggests more focused ways to treat this disease."

The study is published in the journal Nature Genetics.