for example, the rectum, may not of itself be sufficient to confirm absence of polyps. It may be necessary to consider and visually examine other possible parts of the intestinal tract. Colonoscopy is preferred over sigmoidoscopy for this, as it provides better observation of the common right-side location of polyps. The genetic determinant in familial polyposis may also predispose carriers to other malignancies, e.g., of the duodenum and stomach (particularly ampullary adenocarcinoma). Other signs that may point to FAP are the development of Gardner fibromas and desmoid tumors (benign skin tumors that may be apparent before other signs of FAP), pigmented lesions of the retina ("CHRPE—congenital hypertrophy of the retinal pigment epithelium"), jaw cysts, sebaceous cysts, and osteomata (benign bone tumors). The combination of polyposis, osteomas, fibromas and sebaceous cysts is termed Gardner's syndrome (with or without abnormal scarring). Genetics Familial adenomatous polyposis can have different inheritance patterns and different genetic causes. When this condition results from mutations in the APC gene, it is inherited in an autosomal dominant pattern, which means one copy of the altered gene is sufficient to cause the disorder. The incidence of malignancy in these cases approaches 100%. In most cases, an affected person has one parent with the condition. APC gene mutation variants The APC is a tumour suppressor gene responsible for the production of adenomatous polyposis coli (APC), a large multifunction tumour-suppressing protein which acts as a "gatekeeper" to prevent development of tumours. (APC regulates β-catenin, a protein that plays a crucial role in cell communication, signalling, growth, and controlled destruction, but which left uncontrolled also gives rise to numerous cancers). A flaw in the APC gene means APC is not as effective as it should be, and over time it is likely that some cells that should have been controlled by APC will not be, and will instead continue to develop and become cancerous. In familiar polyposis they usually manifest as polyps—small abnormalities on the surface of the intestinal tract. Although the polyps are inherently benign, the first step of the two-hit hypothesis has already taken place: the inherited APC mutation. Often, the remaining "normal" allele is mutated or deleted, accelerating generation of polyps. Further mutations (e.g., in p53 or kRAS) to APC-mutated cells are much more likely to lead to cancer than they would in non-mutated epithelial cells. The normal function of the APC gene product is still being investigated; it