MYC and breast cancer: To gain better insight into the levels of Myc tolerated by breast cancer cells over the long term with and without the addition of an exogenous survival signal, we took advantage of the fact that our 67NR-Myc-RFP cell population exhibits a broad range of Myc expression levels because the conditional Myc transgene (SI Appendix, Fig. S3C) inserts in varied locations and copy numbers in the genomes of the recipient population of 67NR cells.