MYC and breast cancer: Genomic DNA of mammary tumors from Lenti-sgPten-Myc and Lenti-sgRb1-Myc injected WB1P-Cas9 mice showed extensive modification of the target gene (Fig. 2g; Supplementary Figure 4A-B), with a strong bias towards indels resulting in frameshift mutations, supporting homozygous inactivation of the tumor suppressor genes.