Targeting mutant KRAS with CRISPR-Cas9 blocks tumor growth in vivo. Cancer cells were transduced with a lentiviral vector expressing the 35T9P17 sgRNA in a doxycycline-inducible manner and a lentiviral vector that constitutively expresses Cas9. Transduced cells were subcutaneously injected into immunodeficient mice to allow tumor formation over 14 d, after which the mice were treated with doxycycline to induce expression of 35T9P17. Tumor growth was monitored for 12 d. (Hetero.) Heterozygous, (Homo.) homozygous. (A,C) Tumor growth curves. Red and blue lines represent doxycycline-treated and untreated (control) groups, respectively. (B,D) Representative photographs of the mice after 12 d of doxycycline treatment (26 d after tumor cell injection). (E–H) Representative photographs (E,F) and weights (G,H) of tumors removed from euthanized mice after 12 d of doxycycline treatment. Error bars represent SEM. (*) P < 0.05, (**) P < 0.01, (***) P < 0.001.