An attenuated form of familial adenomatous polyposis coli, AAPC, causes relatively few colonic polyps, but still carries a significant risk of colon cancer. The mutant alleles responsible for this attenuated phenotype have been mapped in several families to the adenomatous polyposis coli (APC) locus on human chromosome 5q. Four distinct mutations in the APC gene have now been identified in seven AAPC families. These mutations that predict truncation products, either by single base pair changes or frameshifts, are similar to mutations identified in families with classical APC. However, they differ in that the four mutated sites are located very close to one another and nearer the 5' end of the APC gene than any base substitutions or small deletions yet discovered in patients with classical APC.