After analyzing more than 18,000 genes in breast and colorectal cancer, researchers have linked 300 genes to the two diseases, but only a handful of them are "high-frequency" or commonly occurring genetic mutations.
The majority of mutations appear very infrequently and vary widely from one individual to another, but the gene hunters believe they can be grouped into clusters according to the biochemical processes in the body they affect.
For years, geneticists involved in cancer research thought they would find a few core genes driving these deadly diseases, but the emerging picture is much more complex, suggesting that scores of genes are involved.
"This makes treating these cancers more of a challenge," explained Victor Velculescu an associate professor of oncology at Johns Hopkins Kimmel Cancer Center in Baltimore, Maryland.
"It means we will have to tailor drugs to the individual - to target the specific genetic makeup of their cancer. It suggests the future of cancer treatment lies in personalized medicine. The drugs we have now are blunt instruments," he added.
For the study, the Johns Hopkins researchers screened tissue samples taken from 11 breast and 11 colon cancers that were removed from patients after surgery. They compared the DNA of the tumour genes to healthy reference genes.
They found an average of 77 genes were mutated in an individual colon cancer and 81 in breast cancer.
They suspect that only 15 of these are likely to be crucial to the development of the tumour, in terms of influencing whether it spreads to other tissues or organs beyond its original location.
Those core genes will vary from one individual to another, but they are likely to be united by their action on certain pathways, such as the P13K pathway which is essential for cell growth, survival and immune cell activation.
"The hard part used to be finding these mutant genes, now the challenge will be to link them to specific pathways and understand their function," said Velculescu.
The study is published in the journal Science.