About 5 per cent of colorectal cancers are thought to arise in individuals with a strong hereditary susceptibility to the condition. Inherited cancer susceptibility syndromes predisposing to colorectal cancer include familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC). These conditions are inherited as autosomal dominant conditions characterized by early-onset bowel cancer. A larger proportion of familial clustering of colorectal cancer is characterized by a later age at onset of cancer and a less obvious mode of inheritance. Some of these familial cases may be due to less significant mutations in the genes causing FAP or HNPCC (APC and the mismatch repair genes), or other less penetrant susceptibility genes, interacting with environmental factors. Many genes involved in this type of susceptibility may as yet be unidentified.