Gene scans show that colon and rectal tumours constitute a single type of cancer.
A US-funded consortium of scientists compared the genetic codes of cancer cells found in 224 samples of colorectal tumours.
"This finding of the true genetic nature of colon and rectal cancers is an important achievement in our quest to understand the foundations of this disease," the head of the US National Institutes of Health (NIH), Francis Collins, said in a statement.
"The data and knowledge gained here have the potential to change the way we diagnose and treat certain cancers."
The sweep threw up 24 genes where telltale mutations point to a role in either initiating or accelerated colorectal cancer.
They include the genes ERBB2 and IGF2, which are involved in cell proliferation.
In 2008, colorectal cancer claimed 608,000 lives globally, making it the fourth deadliest form of cancer, according to the latest figures posted by the UN's World Health Organisation (WHO).
The study, authored by The Cancer Genome Atlas project, appears in the British science journal Nature.