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FAP is caused by a mutated adenomatous polyposis coli (APC) gene. Two-thirds of all cases are inherited from a parent with a mutant APC gene, and the remaining one-third of cases arise from a spontaneous APC gene mutation. FAP is a relatively rare syndrome, afflicting about one in 10,000 people. Nearly everyone who has a mutation in the APC gene that causes the classic form of FAP will develop colorectal polyps by age 50, unless they take preventive action. However, the age of onset varies. Consider the following: