TP53 and neoplasm: To confirm the inactivation of p53 in the RCAS-Trp53-gRNA-induced tumors, we exposed tumor-bearing mice and tumorspheres to ionizing radiation (IR): neither p53 nor p21, a well-known p53-induced target, were detectable in the Trp53 null tumors as compared to the Cdkn2a null tumors (Fig. 2f, g).