The transgene construct has a loxP-flanked Beta-geo gene driven by the CAG promoter, followed by the SV40 tsA58 large T antigen (cDNA for the SV40 TAg and the 3' portion of tsA58T antigen sequence with the A438V mutation.) In the absence of cre-mediated recombination, Beta-geo is expressed and detected by X-gal staining. Removal of the loxP-flanked Beta-geo results in expression of the tsA58T antigen, leading to temperature-dependent cell-type independent cell proliferation,