The first cancer patients have been recruited to the main phase of Britain's ground-breaking 100,000 Genomes Project which is helping scientists investigate the genetic roots of disease.
They will join other patients with rare inherited diseases who are already part of the research programme, launched by Prime Minister David Cameron in 2012.
The project aims to map 100,000 complete sets of DNA from around 70,000 NHS patients.
Studying the DNA of cancer patients is expected to lead to better ways to predict, prevent and treat the disease. The programme will be instrumental in launching a genomic medicines service for the NHS.
Life sciences minister George Freeman, who announced the development at the Festival of Genomics taking place in London, said: "The recruitment of cancer patients is a significant milestone in the revolutionary 100,000 Genomes Project.
"It will help to unlock our understanding of the causes of this devastating condition, make the UK a leader in genetic research, and provide better diagnosis and more targeted treatment for thousands of NHS patients across the UK."
Patients are being recruited through 13 Genomic Medicine Centres (GMCS), specialist NHS sites selected across the country to deliver the project.
Part of the programme involves pioneering work to extract enough DNA from a tumour that is of sufficient quality to be fully sequenced - a problem no country has solved so far.
This underlines the UK's position as a world-leader in cutting-edge medical technology, said Mr Freeman.
He also announced that the Government was committing a further £250 million to genomics as part of the recent Spending Review.
Among the cancer patients joining the project are Mary Lloyd, 61, and her two sisters Sandra, 54, and Kerry, 46, who were diagnosed with breast cancer within 15 months of each other.
They signed up for the programme through the University of Hospitals of Leicester NHS Trust.
Mary, a former social worker from Northampton, said: "I was diagnosed in 2013 just after I retired. Then it was Kerry in January 2014, who still has children at school. It was horrendous. Then Sandra in February 2015. You just wonder when it's all going to end. We're all keen to take part. Finding out more opens it all up for the rest of our family."
The number of genomes sequenced by the project now exceeds 6,000.
Sir Harpal Kumar, chief executive of Cancer Research UK, said: "It's very exciting news that the first cancer patients are being recruited to the main phase of this ground-breaking project.
"The comprehensive mapping of patients' DNA will reveal a vast amount of information that could help doctors and scientists develop new ways to prevent, diagnose and treat cancer more effectively in the future."
Health Secretary Jeremy Hunt said: "Genomics is the future of medicine and the sequencing of cancer DNA confirms why the UK is a global leader in this field. Over half a billion pounds has been invested in genomics to ensure that NHS patients continue to benefit from the prospect of better diagnosis and better treatments."