TFR-binding affinity of rhTF and hTF in HeLa cells. Different concentrations of 125I-labeled hTF or rhTF were added to confluent HeLa cells and incubated at 4°C for 2 hr. Cells were then washed with cold PBS, solubilized with 1 N NaOH, and the amount of radiolabelled hTF or rhTF in cell lysate was determined. The total cellular protein in the cell lysate was measured, and used to normalize the data to ng of hTF per mg cell protein (“ng/mg cell protein”). Data is represented as average with error bars indicating standard deviation, n = 3.