GREM1 and polyposis: These data show that embryological activation of aberrant epithelial Grem1 expression does not appear to induce an immediate postnatal polyposis phenotype, but the subsequent derangement of appropriate cell differentiation and shedding expands the intestinal epithelial cell population with epithelial folding and the gradual development of invaginating ectopic crypts, with subsequent polyposis which emerges in adult animals (Fig. S1C, D).