Cross-linking experiments. (A) E1 TEPC clones were labeled with iodinated Epo and then cross-linked with DSS in the absence (lane 1) or presence (lane 2) of a 100-fold excess of unlabeled Epo. (B) UT-7 cells (lane 1), NEO-infected clone N1 (lane 2), and Epo-R–infected clone (lane 3) were labeled with iodinated Epo and then cross-linked with DSS in the absence of 100-fold excess of unlabeled Epo. Epo-binding proteins were immunoprecipitated with anti–Epo-R antibodies bound to Sepharose beads. After denaturation, immunoprecipitated products were separated by SDS-PAGE.