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About 15% of patients with colorectal cancer have a positive family history: 5% have hereditary colorectal cancer (hereditary non-polyposis colorectal carcinoma (HNPCC), familial adenomatous polyposis (FAP) or some other hereditary syndrome), while in 10% no clear hereditary pattern can be recognized ('familial colorectal cancer'). In sporadic and in familial intestinal cancer, a demonstrable hereditary predisposition may undoubtedly exist. HNPCC is often characterized by microsatellite instability, i.e. an increased number of short DNA sequences in the DNA indicating a disorder in DNA repair and a mutation in a DNA 'mismatch repair' (MMR) gene. Indicative of hereditary bowel cancer on the basis of such an MMR gene mutation are: (a) presence of bowel cancer in > or = 3 relatives, (b) early age at the time of the diagnosis of 'bowel cancer', (c) multiple primary bowel tumours, (d) uterine cancer in the family and (e) bowel and uterine cancer in a woman. Recent data demand a new subdivision of hereditary bowel cancer, based upon both the clinical picture and the results of DNA-tests. The genetic alterations in colonic adenomas and carcinomas are known to a large extent. In future these insights may be important in clinical practice, such as a more individual determination of the patient's prognosis and accordingly, of the treatment and follow-up.

About 5% of colorectal cancer cases are due to an autosomal dominant genetic predisposition with high penetrance. In this condition, the patient is carrier of a pathogenic gene mutation present in all body cells which can be transmitted to descendants, a so-called germ line mutation. The mutation is usually present in a tumour suppressor gene. Three subgroups of hereditary colorectal cancer can be distinguished on the basis of the clinical characteristics: (a) syndromes without polyposis (mostly hereditary non-polyposis colorectal carcinoma; HNPCC), (b) syndromes with adenomatous polyposis (mostly familial adenomatous polyposis; FAP) and (c) syndromes with hamartomatous polyposis. Recently, the main gene defects which underlie these syndromes were identified. Consequently, it is possible in approximately half the families with HNPCC or FAP in patients with colorectal cancer to demonstrate the causative gene defect and subsequently, by blood testing of healthy relatives to determine who is and is not a carrier of this hereditary condition. Thus, preventive measures can be directed toward family members with a demonstrable high risk of large bowel cancer.