Hypoxia acts in concert with BMP4 and SCF to induce stress BFU-E expansion in vitro. Spleen cells were plated in methylcellulose media containing the indicated factors and grown at 20% or 2% O₂. (A) Stress BFU-Es were scored after 5 days of culture. (B) The average diameter of stress BFU-E colonies was determined. Representative colonies are presented below each condition. The results presented are the average of 2 independent experiments.