The APC gene encodes a tumor suppressor protein that acts as an antagonist of the Wnt signaling pathway. APC promotes rapid degradation of beta-catenin and participates in Wnt signaling as a negative regulator. APC is also involved in other processes including cell migration, cell adhesion, transcriptional activation and apoptosis. Germline defects in this gene cause familial adenomatous polyposis (FAP), an autosomal dominant pre-malignant disease that usually progresses to malignancy. Disease-associated mutations tend to be clustered in a small region designated as the mutation cluster region (MCR) and result in a truncated protein product. Somatic APC mutations are rare in adenocarcinoma of small intestine and further studies are needed to explore the clinical value of these mutations. Results should be interpreted in conjunction with other laboratory and clinical findings.