Zfra regulates cell growth in a biphasic manner. (A) L929 cells were transfected with various amounts of EGFP-Zfra and/or a non-toxic dose of RIP (100 ng/well) by CaPO4. Zfra alone enhanced cell growth at low doses, but induced death at higher doses (n = 8; mean ± standard deviation). RIP blocked the Zfra-mediated cell growth. (B) Similarly, L929 cells were transfected with various amounts of RIP and/or a non-toxic dose of Zfra (60 ng/well). At low levels, RIP had little or no effect on cell growth. When in combination, both Zfra and RIP effectively increased the cell death (n = 8; mean ± standard deviation). Cells were stained with crystal violet. EGFP-Zfra was expressed in approximately 60–70% of total cells, as visualized by fluorescence microscopy.