In family 2, a splice donor site mutation in APC exon 12 (c.1621_1626+7del, p.A517_Q542del) leading to exon 12 skipping was identified in a 53-year-old male patient who developed ≥20 colorectal adenomatous polyps and had a family history of colorectal polyposis and CRC. This evidence concerns the gene APC and colorectal carcinoma.