Four human cancer derived cell lines and murine Eμ-Myc lymphoma cell lines, which had undergone selection for mutations in TP53/Trp53 during their development, were selected and the endogenous mutant TP53/Trp53 gene was inactivated using CRISPR/Cas9 to generate mutant TP53/TRP53 deleted derivatives. The gene discussed is TP53; the disease is cancer.