Colorectal cancer originates in the mucosal tissue that lines the inside of the colon and rectum. It is thought that most cases of colorectal cancer develop progressively from small abnormal growths called polyps. Ninety percent of colorectal polyps are classified as hyperplastic polyps, growths that generally do not become cancerous. The remaining 10 percent of colorectal polyps are classified as adenomatous polyps, or adenomas, and research has shown that these lesions give rise to virtually all cases of colorectal cancer. Because there is evidence that it takes approximately 10 years for an adenomatous polyp less than 1 centimeter in size to transform into an invasive colorectal tumor, most of these polyps should be detectable by screening—and removed—before they can become cancerous. Therefore, regular screening, especially with flexible sigmoidoscopy or colonoscopy, is probably the single most important tool in the medical toolbox for colorectal cancer, since finding and removing precancerous polyps and early cancers is the best way to reduce morbidity and mortality from this disease. Over the years, clinical implementation of these insights has already saved tens of thousands of lives, with a reduction in colorectal cancer deaths by approximately 3 percent per year from 1999 through 2008—a decrease of around 30 percent. Still, in 2012 it is estimated that more than 143,000 Americans will be diagnosed with colorectal cancer and more than 51,000 will die from the disease.