The hypoxia-inducible enhancer of mouse and human Epo gene functions in Hep3B cells but not in P19 cells. Transient expression of reporter constructs in 21% oxygen (N) and in 2% oxygen (H) was assayed. (A and C) P19 cells; (B and D) Hep3B cells. m0.2k, 0.2-kb mouse Epo promoter; mEn, mouse Epo enhancer; En, human Epo enhancer. 0.2k, 1.6k, and 3.5k represent 0.2-kb, 1.6-kb, and 3.5-kb 5′-flanking regions of human Epo gene. Relative light units (RLU; Luc activity) per β-galactosidase activity are shown. Hypoxic inducibility (the ratio of Luc activity in 2% v 21% oxygen) is also indicated. Each value is the mean ± SD of triplicate experiments.