P19 cells possess an intact hypoxia-signaling pathway through HIF-1 activation. Transient expression of reporter constructs in 21% oxygen (N) and in 2% oxygen (H) was assayed. (A) P19 cells; (B) Hep3B cells. WT, wild type; M1, mutant in the HIF-1 binding site 1; M4, mutant in the HIF-1 binding site 2; LEn, LDHA enhancer; LP, LDHA promoter. Relative light units (RLU; Luc activity) per β-galactosidase activity are shown. Hypoxic inducibility (the ratio of Luc activity in 2% oxygen v 21% oxygen) is shown. Each value is the mean ± SD of triplicate experiments.