Survival of transgenic and normal fetal liver CFU-E after Epo deprivation. Erythroid cells from 13-day old fetal livers were maintained in the absence of Epo for the indicated times and then plated in semisolid medium at a concentration of 2 × 10⁵ cells/mL in the presence of 2 U/mL of Epo. CFU-E–derived colonies were scored at day 2. Results are expressed as the average number of colonies of duplicate cultures from two independent experiments.