New research has shed light on arsenic's ability to work as a cancer treatment in patients with leukaemia.
Arsenic is known to be effective in patients with a type of cancer called acute promyelocytic leukaemia, but the reasons for its effect had remained unclear.
Now, scientists from Cancer Research UK have discovered that arsenic helps molecules called SUMO to stick onto leukaemia-related proteins, enabling the cancer-causing proteins to be identified by enzymes and broken down.
The finding is of particular interest as, ironically, arsenic is one of the chemicals found in cigarettes that causes lung cancer.
Dr Lesley Walker, director of cancer information at the charity, commented: "Discovering which molecules are involved in this process is an exciting step forward in understanding this complex paradox - how can a chemical that causes cancer also cure it?
"It's a great piece of science that will hopefully lead to the development of drugs that home in on specific cancer-causing proteins to beat the disease."
The findings are published in the journal Nature Cell Biology.