Here, by mediating transformation on different mouse backgrounds containing mutations in Kras or other common CRC driver genes, we establish that the presence of diverse priming events in the normal mouse intestinal epithelium can change the transformation and clonal-selection landscape, permitting the fixation of strong driver mutations in Apc and Ctnnb1 that are otherwise lost due to negative selection. The gene discussed is KRAS; the disease is colorectal carcinoma.