Colorectal cancer affects the colon (large intestine) or rectum. An inherited cancer syndrome called hereditary nonpolyposis colorectal cancer (HNPCC) is the cause of colorectal cancer in some patients. People with HNPCC have a higher than usual risk for colorectal and several other types of cancer. In HNPCC, colorectal tumors have a DNA abnormality (DNA is the substance that makes up genes) called microsatellite instability. In addition, people with HNPCC frequently have abnormalities (mutations) in two genes known as MLH1 and MSH2. Patients with an MLH1 or MSH2 mutation are more likely to have colorectal tumors with microsatellite instability than are patients without these mutations. Identifying patients with an MLH1 or MSH2 mutation may be important so that they and their family members can be monitored closely for the development of colorectal tumors. The Bethesda guidelines identify conditions that suggest that a patient mi t have HNPCC (for example, colorectal cancer in two successive generations or cancer at a young age). However, many people who meet a Bethesda criterion do not have HNPCC. It would be useful to have a way to identify people with colorectal cancer who should undergo genetic testing for mutations in MLH1 and MSH2.