Effect of the GATA transcription factors on Epo mRNA expression in Hep3B cells. The level of Epo mRNA was measured by competitive PCR as described in the experimental procedures. These results were normalized to the level of Epo mRNA in Hep3B cells incubated in 1% O₂ with transfection of carrier DNA only: (□), 20 μg of DNA (n = 4); (▪), 40 μg of DNA (n = 1).