Binding of ¹²⁵I-Tpo or ¹²⁵I-Epo to BaF3-MplR-EpoR cells. The cells (1.2 × 10⁶) were incubated with ¹²⁵I-Tpo (0.3 nmol/L) or ¹²⁵I-Epo (0.3 nmol/L) in the presence or absence of a 100-fold excess of unlabeled Tpo or Epo for 1 hour at 37°C. The data represent the mean ± SEM of triplicate determinations from one of three similar experiments.