Expression of oncogenic K-ras can be efficiently induced from its endogenous promoter by a Tet-on system. (A) Schematic representation of the tetracycline-inducible system, which involves in breeding 3 lines together. The first line is the M2rtTA transactivator driven from the endogenous Rosa26 promoter (R26-M2rtTA). The second line contains 2 alleles, cre recombinase driven by the Tet operator (LC-1) and a floxed β-galactosidase-neo (βgeo) reporter cassette at the endogenous Rosa26 locus (R26R). The third line is the conditional oncogenic K-ras at its endogenous locus. (B) TER119-negative cells were purified from individual embryos that carried all the alleles and were induced with or without doxycycline (Dox) for 42 to 48 hours. βGeo activity was detected by a fluorogenic substrate fluorescein di-β-D-galactopyranoside (FDG).