WHO histological classification of tumours of the colon and rectum

Colorectal carcinomas vary considerably throughout the world, being one of the leading cancer sites in the developed countries. Both environmental (diet) and genetic factors play key roles in its aetiology. Genetic susceptibility ranges from well-defined inherited syndromes, e.g. familial adenomatous polyposis, to ill-defined familial aggregations. Molecular genetic mechanisms are diverse, and recent data suggest two main pathways: a mutational pathway, which involves inactivation of tumour suppressor genes such as APC; and microsatellite instability which occurs in hereditary nonpolyposis colon cancer (HNPCC) and a proportion of sporadic carcinomas.