32D cells containing the EPO-R reconstitute EPO internalization and receptor downregulation. ¹²⁵I-EPO internalization studies. Solid symbols represent the percentage of total bound counts at time 0 that were internalized. Open symbols represent the percentage of total bound counts at time 0 that remained surface bound. Open symbols and a broken line represent the percentage of total bound counts at time 0 that were released into the medium. Standard deviations for each point were within 10% of the value plotted and are not shown. At each time point, the summation of internalized EPO, surface-bound EPO, and counts in the medium equaled the amount of ¹²⁵I-EPO bound at time 0. Internalization studies were performed on 3 different clones and were performed 2 times for each clone. Data presented are from a single representative clone of each. 32D.Wild-type EPO-R(1-483) cells, squares; 32D.Neo cells (no EPO-R), circles.