Familial adenomatous polyposis (FAP) is an autosomal-dominant disease chara
cterized by the development of hundreds of adenomatous polyps of the colore
ctum. Approximately 80% of FAP patients can be shown to have truncating mut
ations of the APC gene. To determine the cause of FAP in the other 20% of p
atients, MAMA (monoallelic mutation analysis) was used to independently exa
mine the status of each of the two APC alleles. Seven of nine patients anal
yzed mere found to have significantly reduced expression from one of their
two alleles whereas two patients were found to have full-length expression
from both alleles. We conclude that more than 95% of patients with FAP have
inactivating mutations in APC and that a combination of MAMA and standard
genetic tests will identify APC abnormalities in the vast majority of such
patients. That no APC expression from the mutant allele is found in some FA
P patients argues strongly against the requirement for dominant negative ef
fects of APC mutations. The results also suggest that there may be at least
one additional gene, besides APC, that can give rise to FAP.