Mitogenic synergy between c-kit and stably transfected Epo receptor forms in FDC2–wt-ER, FDC2–ER-Bx1 and FDC2-EE483 cells. In experiments aimed at testing the ability of the wt-ER and the receptor forms ER-Bx1 and EE483 to act synergistically with SCF, FDC2-derived cell lines were exposed to SCF, Epo, or both cytokines. Levels of induced mitogenesis then were assayed as above (see Fig 2 and Materials and Methods). In each line, synergy in mitogenic signaling was observed upon Epo and SCF costimulation at all cytokine concentrations tested (SCF, 0, 6.8, 13.5, 27 ng/mL or 0, 0.2, 0.4, 0.8 nmol/L; Epo, 0, 0.11, 0.22 U/mL or 0, 0.022, 0.044 nmol/L; EGF 0, 0.5, 1.0 ng/mL or 0, 0.1, 0.2 nmol/L). Levels of synergy are indexed above histograms, and represent the fold increase in rates of [³H]thymidine incorporation over calculated additive values for Epo and SCF, or EGF and SCF.