The first 14 exons of the APC gene have been screened by the denaturation gradient gel electrophoresis method in 160 unrelated patients with familial adenomatous polyposis coli (APC) syndrome. Four polymorphic variants corresponding to silent mutations not associated with the disease phenotype were observed. Mutations predicted to alter the coding property of the APC gene were observed in 26 patients. All these mutations are expected to lead either to aberrant splicing, to synthesis of a truncated APC protein because of the emergence of a stop codon, or to a change in the translation reading frame. Single-base-pair substitutions were observed on 21 occasions. The most frequent mutation (eight cases) was a C-to-T change which exclusively occurred on the nontranscribed strand within a CG dinucleotide.