Regulation of HIF-1α under hypoxic conditions. HIF-1α expression at 3% O2 was determined after 1 h, 3 h, 1 d, 2 d, 3 d and 4 d using western blot analysis. The cells were differentiated under hypoxia up to 4 days. Application of 10 IU/ml EPO is indicated by (+). Untreated conditions are indicated by (-). GAPDH was used as loading control. "Prol" is indicating proliferating cells, representing time point zero of differentiation.