UK scientists have uncovered a vital clue to stopping cancers spreading around the body.
Cancer cells can spread through the body
A protein called Tes is able to block a second protein, Mena, from helping cancer cells "crawl" away from the initial tumour.
The London Research Institute team says this knowledge should help in the design of new drug treatments to anchor a tumour in one site.
The work is published in the latest edition of the journal Molecular Cell.
The Mena protein is found in excessive amounts in tumours and was already known to help cancer cells move away from a tumour and spread around the body to form secondary cancers - one of the main obstacles in treating cancer.
Study leader Dr Michael Way said Tes was not as well studied but in many tumours it is absent.
Using a range of techniques, including X-ray crystallography, which can be used to determine the 3-dimensional structure of a molecule, Dr Way and his colleagues found that Tes attached itself to Mena in such a way it could no longer bind with other proteins.
Without being able to interact with its normal binding partners, Mena was no longer able to help the cancer cells migrate from the tumour.
Figures show about 20,000 people have died from cancer every day across the world in 2007.
And one in three people will be diagnosed with cancer at some point in their lifetime.
Scientists are gradually developing a deeper understanding of the causes of cancer, investigating the complex interaction of chemicals, genetics, ageing and diet.
Dr Way said if researchers could design a drug to block Mena in the same way as Tes, it would potentially be a way to stop the spread of cancer once a tumour had formed.
"What was surprising was, when you look at Tes you wouldn't predict it would interact with Mena.
"Looking at the structure gives us clues in designing drugs which mimic the interaction with Tes and prevent cells from migrating, although that's a long way away."
He said Mena was a very small part of the spread of cancer cells, but that was one of the control mechanisms that goes wrong.
Dr Lesley Walker, director of science information at Cancer Research UK, said: "Cancer cells use many complex processes when they break away from their tumour and spread to other areas of the body.
Understanding these mechanisms and increasing our knowledge about this protein can hopefully help us to develop more effective cancer treatments in the future."