Scientists have pinpointed a new cancer-causing agent - tiny pieces of genetic material called microRNAs.
Analysis could distinguish between cancer types
Studies in the journal Nature suggest identifying the fragments may help detect even hard-to-diagnose cancer.
New York's Cold Spring Harbor Laboratory found raised levels of microRNA activity were linked to aggressive forms of blood cancer.
A second study found different cancers can be identified by their highly distinct patterns of microRNA activity.
It is thought the discovery could lead to an overhaul of the way cancers are classified, diagnosed and treated.
Dr Paul Meltzer, of the US National Human Genome Research Institute, said: "These studies change the landscape of cancer genetics."
MicroRNAs appear to regulate a broad array of physiological and developmental processes.
However, they are so tiny, and prone to subtle variation that it has proved difficult to pin down their exact role.
More than 200 different microRNAs have been identified - but the function of only a handful has been established.
Despite this scientists suspected they are implicated in cancer because of their role in embryonic development.
The New York laboratory team tested the theory by examining blood cancer (lymphoma) cells.
The cells showed increased levels of activity in a genetic segment containing a cluster of microRNAs called mir-17-92.
An analysis of tumour samples taken from patients produced similar results.
The researchers next bred mice with increased activity in the key cluster area.
They developed lymphoma tumours more quickly than normal animals, and the tumours were more aggressive, killing all the animals within three months.
The researchers believe the microRNA activity may block the usual pattern of programmed cell death within a tumour, speeding up its growth and making it highly malignant.
Researcher Dr Gregory Hannon said: "This is by no means a final answer about the role of microRNAs in cancer.
"But it's the first really definitive link where we can show with biological experiments that microRNAs can act as an oncogene [cancer gene]."
In separate research, a team led by Dr Todd Golub, of the Dana-Farber Cancer Institute, developed a sophisticated colour-coding technique which enabled them to analyse microRNAs more effectively than has previously been possible.
They examined different types of cancer cells to determine whether they showed distinct patterns of microRNA activity.
The patterns proved to be so distinctive that the researchers were able to use them to distinguish between not only normal cells and cancer cells, but different types of cancer, and even different subtypes of the same cancer.
The technique proved particularly useful in identifying cancers than can be difficult to distinguish from other forms because they look so similar under the microscope.
Henry Scowcroft, of the charity Cancer Research UK, said RNA had often been thought of as the "disposable" sister molecule of DNA - but recent research suggested it was nothing of the sort.
"Although at a very early stage, the research may open up a whole new world of discovery for cancer researchers.
"If microRNAs are as important in the everyday workings of cells as this research suggests, then exploiting them could play a role in almost every aspect of cancer medicine, from screening and diagnosis, to prediction of treatment outcomes, to cancer treatment itself."