Colorectal cancer is cancer of the colon (large intestine) or rectum. Some colorectal cancer is caused by an inherited cancer syndrome called hereditary nonpolyposis colorectal cancer (HNPCC). People in families with HNPCC have a higher than usual risk for colorectal and some other cancers. Genetic testing to detect a predisposition for hereditary colorectal cancer is now possible, but testing for the gene abnormalities (mutations) associated with HNPCC is too expensive to be done on all patients with colorectal cancer. Consequently, the National Cancer Institute (NCI) has published guidelines for looking for HNPCC in patients with new diagnoses of colorectal cancer. The guidelines advise testing only patients who meet clinical criteria that suggest a high risk for HNPCC (for example, one or more first-degree relatives with colorectal cancer or one of the other HNPCC-associated cancers, or young age at diagnosis) and whose tumors also show an abnormality in the DNA known as microsatellite instability. Most olorectal tumors in HNPCC (85% to 90%) have microsatellite instability. However, many patients who have tumors that show microsatellite instability do not have HNPCC. The guidelines recommend doing further genetic testing for HNPCC in patients who meet the clinical criteria and also have microsatellite instability. The reason to look for HNPCC is to identify families that need close screening. The cost-effectiveness of this strategy is not known.