The authors describe 2 extended kindreds supposedly manifesting familial multiple adenomatous polyposis coli (FPC), but which show marked heterogeneity in the phenotypic expression of colorectal adenomatous polyps. In one family, 2 individuals had diffuse polyposis at very early ages (7 and 10 years), while 6 others (aged 23 to 72 years) had solitary polyps only. Of the patients with solitary polyps, 2 had associated colonic malignancies (ages 26 and 35), while another had a prophylactic colectomy performed at age 46. In the second family, 5 of the 11 patients with evidence of polyps showed the classical presentation of FPC, while the remainder showed marked phenotypic variation. The marked variability in frequency and location of colon polyps points to the need to reassess our traditional criteria for diagnosis of FPC. The high risk of early onset colon cancer in patients from these families who have the most minimal manifestation, namely isolated polyps, recommends more careful scrutiny of supposedly unaffected members of all FPC kindreds.
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