Sequencing analysis, RT-PCR analysis, and MLPA analysis of the APC genes of 8 Japanese (A)FAP patients from 8 unrelated families revealed a nonsense mutation, a frameshift mutation, or an exonic mutation leading to abnormal splicing, all of which resulted in the production of a truncated APC protein, in every patient. This evidence concerns the gene APC and Familial adenomatous polyposis.