Colorectal cancer (CRC) is the third most common cancer and the second leading cause of cancer-related deaths in the United States. The 5-year survival rate is approximately 90% for CRC patients with localized disease (stage I or stage II). However, the average 5-year survival rate drops to 68% for patients with CRC that has spread to the lymph nodes (stage III) and to 10% for patients with distant metastases (stage IV). Thus, the staging of colorectal tumors is a critical factor in assessing prognosis. In addition, tumor staging is an important factor in determining the most appropriate treatment for a given patient, including whether or not the patient is a good candidate for adjuvant chemotherapy. Accurate CRC staging depends on an adequate sampling and histopathological analysis of regional lymph nodes. However, traditional histopathology typically examines a very small portion of patient specimens. As a result, it is believed that some CRC patients harbor micrometastases in their lymph nodes that remain undetected after histopathological analysis. Consistent with this, up to 30% of CRC patients with apparently localized CRC (i.e., with negative lymph nodes by standard analysis) experience disease recurrence. Because of the limitations of routine histopathology in CRC staging, recent pharmacogenetic studies have been performed in order to identify specific biomarkers that may allow for the identification of patients with occult micrometastases who are at an increased risk for disease recurrence and may benefit from adjuvant chemotherapy. Expression of the gene encoding guanylyl cyclase, GCC (also known as guanylyl cyclase 2C, or GUCY2C), has been evaluated as a biomarker used to augment the traditional staging techniques for CRC. Expression of the GCC gene, which is located on chromosome 12 at band p12, appears to have a high specificity for colorectal tumor cells in extraintestinal tissues. As a result, GCC expression may serve as a marker to indicate the presence of metastatic lesions not detected by routine histopathology. As such, GCC expression may help identify CRC patients with occult metastases who would be at an increased risk of disease recurrence, and who may benefit from adjuvant chemotherapy.