Affinity cross-linking with ¹²⁵I-Tpo. BaF3-MplR cells (A) or normal human platelets (B) were incubated with ¹²⁵I-Tpo (0.6 nmol/L) in the presence or absence of a 100-fold excess of unlabeled Tpo and cross-linked with 1 mmol/L BS³. Cell lysates were analyzed by SDS-PAGE and autoradiography. The upper band represents the major ¹²⁵I-Tpo-Mpl receptor complex and the lower band is free ¹²⁵I-Tpo. Lane 1, ¹²⁵I-Tpo; lane 2, ¹²⁵I-Tpo plus unlabeled Tpo. Molecular weight markers are indicated.