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Hundreds of mutations have been found throughout the APC gene. It has long been recognized that some families with APC mutations have different symptoms than others. Studies comparing symptoms in patients with different and similar APC mutations have been done to see if there are any correlations.
A correlation between specific mutations and symptoms is called a "genotype-phenotype" correlation. Genotype-phenotype correlation studies for the APC gene have shown that the location of a mutation in the gene provides some information about the types of FAP health problems a person will have. For instance, mutations in certain parts of the gene are associated with an increased rate of desmoid tumors (locally invasive tumors that grow aggressively and can be life threatening), osteomas (benign bony growths), and epidermoid cysts (skin lesions). Where in the gene a mutation lies also provides some information about polyp burden (the number of polyps a person will develop).
Even though some correlations exist, there is often variability of symptoms between people who have the same mutation. This is because factors other than the APC mutation (environmental factors, other genetic factors) contribute to the development of polyps and cancer.