Overexpression of IL5R does not increase the mitogenic activity of IL-5. Rapid growing TF-1, JYTF-1, TF1, and TF8 cells were cultured in IL-5–containing (▧) or cytokine-free (□) media for 24 hours and then 1 × 10⁴ viable cells were subjected to [³H]thymidine pulse-labeling assay as described in Materials and Methods. For each sample, data shown is the average of 13 to 17 independent measurements (n).