We report the cases of two familial adenomatous polyposis (FAP) families who presented with the same 2 base pair deletion (AG) at codon 1465 of the adenomatous polyposis coli (APC) gene, but showed phenotypic variability. The mutation was revealed by a simple nonradioactive method using a heteroduplex analysis and identified by a sequence analysis. This observation suggests the responsibility of modifier genes in FAP patients' phenotype.