Clinical management of colorectal cancer patients is m odified by the discovery of a family predisposition.

Recent advances in molecular genetics made it possible to identifyseveral genes responsible for familial aggregation of colorectal cancers.Among those, genetic alterations of hMSH2 and hMLH1 genesare responsible for HNPCC syndrome (Hereditary Non PolyposisColorectal Cancer) and genetic alterations of APC gene for FamilialAdenomatosis Polyposis (FAP). These two syndromes areresponsible for 3 to 5 % of diagnosed colorectal cancers each yearin France. If the recognition of colorectal cancers occurring withinFAP is most of the time easy, the recognition of cancers developingin HNPCC syndromes is more difficult. The characterization ofmicrosatellite instability tumour phenotype is of an invaluable helpfor the diagnosis of HNPCC syndrome, apart from the characteristicfamily forms. The clinical management of these patients is modifiedby the high risk of a second colorectal cancer associated and endometrialcancer for women. In addition, these cancers have a betterprognosis and are more sensitive to chemotherapy compared tosporadic colorectal cancers.