A key issue in cancer biology is whether genetic lesions involved in tumor initiation or progression are required for tumor maintenance. This question can be addressed with mouse models that conditionally express oncogenic transgenes, i.e., under the control of tetracycline (tet)-dependent transcriptional regulators. We have developed a system for studying tumor maintenance by using avian retroviral [i.e., replication-competent avian leukosis virus long terminal repeat with splice acceptor (RCAS)] vectors to deliver the reverse tet transcriptional transactivator (rtTA) gene to somatic mammalian cells. rtTA can regulate any transgene in which the protein coding sequence is preceded by a tet-operator (tet-o); RCAS viruses infect only cells engineered to express ectopically the avian retroviral receptor, TVA. One vector, RCAS-rtTA-IRES-GFP, also encodes GFP to identify infected cells. Infection of cells from beta-actin TVA transgenic mice with this vector permits efficient regulation of tet-responsive transgenes. Sarcomas arise when p53-deficient murine embryonic fibroblasts carrying beta-actin TVA and tet-o-K-ras4bG12D transgenes are infected with RCAS-rtTA-IRES-GFP and introduced into nude mice treated with the tet analog, doxycycline (dox); when dox is withdrawn, K-ras4bG12D levels fall, cells undergo apoptosis, and tumors regress. Regression can be prevented by means of a genetic complementation assay in which tumors are superinfected before dox withdrawal with other RCAS viruses, such as those carrying an active allele of K-ras. Many TVA and tet-regulated transgenic mice have been generated; thus, this method for somatic cell-specific and temporally controlled gene expression may have broad applications for the study of oncogenesis and tumor maintenance, as well as other cell functions and development.