Proliferative response and ligand binding by ΔCRM-1 and CRM-2/CRM-2 cells. (A) BaF3, Mpl-FLAG, ΔCRM-1, and CRM-2/CRM-2 cells were grown either in the absence of growth factor (□) or presence (▪) of recombinant murine Tpo 30 ng/mL for 5 days. All values were determined in triplicate. Error bars represent SEM. (B) Binding of¹²⁵I-Tpo by BaF3, Mpl-FLAG, ▵CRM-1, and CRM-2/CRM-2 cells. Cells were incubated with ¹²⁵I-Tpo in the absence (▪) or the presence (□) of excess unlabeled 70-kD Tpo. The values represent mean of triplicate measurements. Error bars indicate standard error of the mean. Similar results were obtained in two additional experiments.