Canadian researchers have discovered that a single genetic mutation is behind one of the deadliest forms of ovarian cancer.
The groundbreaking discovery centres around the rare and often untreatable "granulosa cell tumours" but could help unravel the causes of a host of other cancers.
The team used new state-of-the-art computer technology to pinpoint the one "spelling mistake" in part of the ovarian cancer's DNA.
Dr David Huntsman, a genetic pathologist of British Columbia University, said the find described as a "Eureka" moment shows the power of new DNA sequencing technology.
By identifying the singular mutation that causes granuloma cell tumours, we can now more easily identify them and develop news ways to treat them," he added.
In the past when scientists wanted to look at the sequence of a tumour it was a laborious and expensive process with each gene individually decoded.
But such has been the rapid development of technology and computing power that it is now feasible and cost effective.
Dr Marco Marra, of the Genome Sciences Centre in British Columbia in Canada, said: "This task would have been unfathomable in terms of both cost and complexity even two years ago."
The researchers, whose findings are published in the New England Journal of Medicine, used "next generation" sequencing machines that can decode billions of strands at rapid speed and new computer techniques to quickly assemble the data.
It marks the beginning of a new medical era with the exposure of the complete genetic sequence of cancers and the variants that cause them.
For women with granuloma cell tumours it represents the first specific diagnostic tool and the development of much needed new therapies.
The researchers decoded four samples of granuloma cell tumours which affect five per cent of ovarian cancer patients - and found a single nucleotide located in the FOXL2 gene was mutated in every one.
They then confirmed their findings by comparing them with a large number of similar samples from across Canada and other countries.
Most types of cancers, including ovarian ones, have a broad range of genetic abnormalities so the characteristic single DNA mutation in granuloma cell tumours can serve as an easy to read identity tag for this type.
Dr Hunstman said: "Based upon our success in decoding granuloma cell tumours we are focusing on other rare tumours in what could be described as a guerrilla war on cancer.
"We hope that these studies will not only help future patients with rare tumours but will also teach us about more common ones as well."
Professor Michael Birrer, a leading cancer expert at Harvard Medical School, said he was "excited" by the findings.
"This tour de force study reveals the power of genomic approaches to cancer, particularly rare tumours," he said.