APC and colon adenoma: Using this system, it was shown that the expression of a mutant KRAS allele into mouse colonic cells induced the formation of lesions reminiscent of human hyperplastic polyps, but failed to induce the expansion of the intestinal stem cell population; in contract, inactivation of both APC alleles resulted in the expansion of the crypt stem cell population, generating an adenomatous epithelium resembling human colon adenomas [277].