Epo-dose response curves from adult bone marrow transgenic CFU-E and BFU-E progenitors. Transgenic (closed symbols) and control erythroid progenitors (open symbols) were cultured in the presence of the indicated concentrations of recombinant Epo. CFU-E–derived colonies were scored at day 2 (A) and BFU-E–derived colonies were scored at day 7 (B). Results are expressed as the average number of colonies per 5 × 10⁵ cells plated.