GREM1 and Cowden disease: Although we additionally searched for germ line mutations in other genes that predispose to colorectal polyps such as SMAD4 and BMPR1A (juvenile polyposis syndrome), STK11 (Peutz-Jeghers syndrome), GREM1 (hereditary mixed polyposis syndrome), and PTEN (Cowden syndrome), all variants in these genes were not deleterious but non-pathological SNPs (Supplementary Table S1).