Scientists have discovered how arsenic works as an effective treatment for leukaemia - according to a report in Nature Cell Biology today.

Patients with a certain kind of leukaemia (acute promyelocytic) can be successfully treated with arsenic, but how the process worked was unknown.

Now the mystery is solved, more targeted treatments with fewer side effects are likely to be developed.

Lead author, Cancer Research UK's Professor Ronald Hay based at the University of Dundee, said: "Our discovery is key to understanding how we can enhance the anti-cancer properties of this poison."

"Knowing the specific molecules involved allows us to now work on creating more targeted and effective cancer drugs with fewer side effects."

The scientists watched the drug at work in animal cells. They modified some cells to remove certain proteins and discovered the drug had different effects.

They found that arsenic helps molecules called SUMO stick onto proteins involved in leukaemia. An enzyme called RNF4 hunts down SUMO and breaks down the cancer-causing proteins.

Discovering which molecules are involved in this process is an big step towards understanding the paradox of how a chemical that causes cancer can also cure it.

The research may lead to the development of drugs that home in on specific cancer-causing proteins to beat the disease.