Survival and proliferation of factor-dependent HCD57 cells in response to in vitro stimulation by SCF isoforms or by a combination of SCF isoforms and rhEPO. HCD57 cells were factor-starved, and stromal cells expressing various forms of SCF were mitomycin C–treated as described in Materials and Methods. HCD57 cells were then plated at 1 × 10⁴ cells per mL on day 1 (Input cells) in medium without growth factor (No Epo), supplemented with rhEPO (1 μ/mL Epo), or cocultured with parental Sl/Sl4 cells (Sl/Sl4) or with stromal cells expressing either the soluble (Sl/Sl⁴-S) or the MR (Sl/Sl⁴-MR) form of SCF alone or in combination with rhEPO (Sl/Sl⁴-S + 1 μ/mL rhEPO or Sl/Sl⁴-MR + 1 μ/mL rhEPO). Viable cells were counted after 48 hours of culture. Data represent the mean ± SEM (bars) for each group on one of the two representative experiment done in triplicate. (*)P < .05 Sl/Sl⁴-MR versus Sl/Sl⁴-S and (*)P < .05 Sl/Sl⁴-MR + 1 μ/mL rhEPO versus Sl/Sl⁴-S + 1 μ/mL rhEPO.