KRAS and neoplasm: In fact, the prevalence of a different Kras mutation in urethane-induced tumors (KrasQ61R) compared to KRASG12C/D mutations in the transgenic mouse models has led to the suggestion that chemical carcinogens introduce KRAS mutations in a different population of tumor-initiating cells than mouse models of genetic KRAS activation (Westcott et al., 2015).