PURPOSE: Familial adenomatous polyposis, an autosomal-dominant disease characterized by the presence of 100 or more colorectal adenomatous polyps, results from mutations in the adenomatous polyposis coli tumor suppressor gene. This study was designed to investigate adenomatous polyposis coli gene mutations in members of Turkish families with familial adenomatous polyposis to constitute an adenomatous polyposis coli mutation spectrum for the Turkish population and to determine specific biomarkers for use in the early diagnosis of familial adenomatous polyposis. METHODS: We investigated adenomatous polyposis coli gene mutations in six unrelated families with familial adenomatous polyposis by using heteroduplex analysis and DNA sequencing. RESULTS: We identified three different mutations in six families. Of these one is known and two are novel: 1018T>C and 1309delGAAAA. The mutation of a T to C transversion at codon 1018 does not cause an alteration in the meaning of the codon; however, it was determined that this silent mutation does cause the formation of new exonic splicing enhancers (ESEs) motifs on a mutated sequence by using ESEfinder program. CONCLUSIONS: This study contributes to enlarging the adenomatous polyposis coli gene mutations spectrum and to defining new biomarkers for the early diagnosis of Turkish patients with familial adenomatous polyposis. © 2007 American Society of Colon and Rectal Surgeons.