A comparison of the biological activity of wt-SCF²⁴⁸ and wt-SCF²²⁰ withSl^17H-SCF²⁴⁸ andSl^17H-SCF²²⁰ isoforms of SCF. G1E-ER2 cells were cocultured for 24 to 48 hours with parentalSl/Sl4  stromal cells or (A) stable stromal cell transfectants expressing wt-SCF²⁴⁸ orSl^17H-SCF²⁴⁸ SCF or (B) stable stromal cell transfectants expressing wt-SCF²²⁰ orSl^17H-SCF²²⁰ SCF. Proliferation was measured by thymidine incorporation assay. Results show the mean ± SEM of a representative experiment performed at least twice with more than 1 clone in replicates of 6. *P < .05Sl/Sl4 v wt (SCF²⁴⁸ and SCF²²⁰); **P < .05 Sl/Sl4 v Sl^17H (SCF²⁴⁸ and SCF²²⁰); and P < .05 wt (SCF²⁴⁸ and SCF²²⁰) v Sl^17H(SCF²⁴⁸ and SCF²²⁰). Note difference in scale between (A) and (B).