Germline mutations in the exonuclease (proofreading) domains of 2 DNA polymerases (POLE and POLD1) have been associated with a dominantly inherited, highly penetrant syndrome of oligo adenomatous polyposis and early-age-of-diagnosis colorectal cancer and endometrial cancer. The loss of proofreading capability causes multiple mutations throughout the genome and is manifest as microsatellite-stable, chromosomal unstable cancers. This is an important addition to the range of dominantly inherited syndromes of colorectal cancer predisposition.