Scientists say they have taken a big step towards blocking a chemical vital to the growth of many cancers.
They have unpicked the structure of telomerase, an enzyme which, when active, helps keep cells in an "immortal" state.
The chemical is at work in more than nine out of ten types of tumour.
Researchers from Philadelphia's Wistar Institute, writing in the journal Nature, say their efforts could lead to drugs which switch it off.
All cells in the body have a natural clock - the telomeres - which shorten every time the cell divides.
"The idea is that you could convert immortal cancer cells back into mortal ones by blocking telomerase in this way"
This change is responsible for changes within the ageing body, as cell division slows down.
Some cells, such as stem cells within the embryo, use the chemical telomerase to maintain telomere length.
Many tumours have hijacked the telomerase system to fuel their uninhibited growth.
The Wistar team has found a new way to map the structure of the most active part of the chemical.
'Few side effects'
Dr Emmanuel Skordalakes said that this detailed picture would help provide molecular targets for drugs.
"Telomerase is an ideal target for chemotherapy because it is active in almost all human tumours, but inactive in most normal cells.
"That means that a drug that deactivates telomerase would likely work against all cancers, with few side effects."
Professor Rob Newbold, from Brunel University in Uxbridge, said it was a "very important" achievement.
"Telomerase controls the evolution of cancers - and is a key characteristic of human cancer cells.
"The idea is that you could convert immortal cancer cells back into mortal ones by blocking telomerase in this way.
"Having discovered the structure now, it will certainly help the development of drugs."
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