Transferrin receptor expression is independent of iron in differentiating mouse erythroblasts. Fetal liver–derived mouse erythroid progenitors pretreated as described in the legend to Figure 2 were analyzed for transferrin receptor (TfR1) expression. (A) TfR1 mRNA determined by Northern blotting (28S rRNA hybridization as control); (B) total TfR1 protein determined by Western blotting; Erk1/2, loading control. (C) TfR1 cell-surface expression was determined by flow cytometry.