Conditionally active forms of the Raf proteins (Raf-1, B-Raf, and A-Raf) were created by ligating NH 2 -terminal truncated activated forms (#) to the estrogen receptor (ER) hormonebinding domain resulting in estradiol-regulated constructs (#Raf:ER). These different Raf:ER oncoproteins were introduced into the murine FDC-P1 hematopoietic cell line, and cells that grew in response to the three #Raf:ER oncoproteins were isolated. The ability of FDC-P1, #Raf-1:ER, #A-Raf:ER, and #B-Raf:ER cells to form tumors in severe combined immunodeficient mice was compared. Mice injected with #Raf:ER cells were implanted with B-estradiol pellets to induce the #Raf:ER oncoprotein. Cytokine-dependent parental cell lines did not form tumors. Implantation of B-estradiol pellets into mice injected with #Raf:ER cells significantly accelerated tumor onset and tumor size. The recovered #Raf:ER cells displayed induction of extracellular signalregulated kinase (ERK) in response to B-estradiol stimulation, indicating that they had retained conditional activation of ERK even when passed through a severe combined immunodeficient mouse. The #Raf:ER cells were very sensitive to induction of apoptosis by the mitogen-activated protein/ERK kinase (MEK) 1 inhibitor CI1040 whereas parental cells were much less affected, demonstrating that the MEK1 may be useful in eliminating Ras/Raf/MEK-transformed cells. Furthermore, the effects of in vivo administration of the MEK1 inhibitor were evaluated and this inhibitor was observed to suppress the tumorigenicity of the injected cells. This #Raf:ER system can serve as a preclinical model to evaluate the effects of signal transduction inhibitors which target the Raf and MEK proteins. (Cancer Res 2005; 65(21): 9962-70)