Familial adenomatous polyposis (FAP), Gardner syndrome (polyposis, osteomas and epitheliomas), flat adenoma syndrome (attenuated APC) and Turcot syndrome (colorectal polyposis with brain tumours) are distinctive clinical syndromes. Each is caused by mutations in the adenomatous polyposis coli (APC) gene on chromosome 5q21, although Turcot syndrome may have other causes. A variety of APC mutations are recognized, which can be associated with the character and severity of the clinical syndromes.