Oxygen-independent expression of Epo mRNA by P19 cells. P19 cells were cultured in 21%, 5%, and 2% oxygen for 24 hours. Total RNA was prepared and competitive RT-PCR was performed to estimate Epo mRNA. Competitor DNA in the PCR reaction mixtures was increased from 0 to 100 fg.41 The amplified products of 451 bp and 282 bp were derived from transcribed Epo cDNA and the competitor DNA, respectively. The band intensity was measured and Epo mRNA was calculated from the equivalence points of the band intensity of the amplified products. The calculation yielded 0.69 ng Epo mRNA/mg total RNA when the cells were cultured in 21% oxygen, 0.48 in 5% oxygen, and 0.78 in 2% oxygen. The content of Epo mRNA relative to that in 21% oxygen is shown.