Scientists have identified a gene that is strongly associated with an increased risk of childhood asthma.
Asthma is common in the UK
The team of international researchers hope their work, published in Nature, will lead to new treatments.
Studying more than 2,000 children, they pinpointed a gene called ORMDL3, which was found at higher levels in the blood cells of children with asthma.
Carrying a specific variant of this gene may increase the risk of developing asthma by up to 70%.
The researchers also identified genetic markers on chromosome 17 which appeared to alter levels of ORMDL3.
The combination of genetic and environmental factors which cause asthma has been poorly understood.
Researcher Dr Miriam Moffatt, from Imperial College London, said "We are confident that we have discovered something new and exciting about childhood asthma.
"These novel findings do not explain completely how asthma is caused, but they do provide a further part of the gene-environment jigsaw that makes up the disease."
Her colleague, Professor William Cookson, said the results provided the strongest genetic effect on asthma so far discovered.
However, he said it remained unclear how ORMDL3 increased the risk of asthma.
He said: "Similar genes are found in primitive organisms such as yeast, so we suspect that ORMDL3 may be a component of quite ancient immune mechanisms. It does not seem to be part of the allergic process."
Asthma is the most common chronic disease of childhood.
The scientists reached their conclusions after comparing the genetic makeup of 994 patients with childhood onset asthma and 1,243 non-asthmatics.
They looked at mutations in genetic building blocks, called nucleotides, which make up DNA.
There are mutations in around one in every 600 nucleotides and the scientists examined more than 317,000 of these mutations to find those specific to childhood asthma.
They also looked at how genes were being expressed within human blood cells.
Dr Victoria King, of the charity Asthma UK, said: "This is an exciting development in determining how genetics affects the chance of developing asthma in childhood, which currently affects one in 10 children in the UK.
"Through research like this, it will be possible to determine both the risks and protective factors associated with a person's genetic makeup, with the long-term aim of preventing and treating asthma in both childhood and later in adult life."