Decreased Tfr expression in the absence of STAT5A/B. Transferrin receptor expression in control and Stat5a/b mutant Ter119-positive cells and (A-C) transferrin receptor mRNA expression in Stat5a/b^{f/f; TC} and control adult mice (D). (A,B) Gated Ter119-positive cells were analyzed for fluorescence of an α-CD71 (Tfr1) antibody in fetal liver and adult bone marrow and spleen cells. Peripheral blood data were generated by double staining with the α-CD71 antibody and thiazole orange dye, which stains RNA and can be used to isolate reticulocytes of a narrow age range as the amount of RNA changes significantly during maturation. (A) Representative data from 8 adult peripheral blood analyses. Top panels show the gate for thiazole orange–positive cells (right) in the whole red blood cell population (left panel). (B) Quantification of Tfr1 protein levels, expressed as the median measurements of α-CD71 fluorescence. E14 data were generated from Stat5a/b^−/− embryos. n = 10 in each group. Adult data were generated from Stat5a/b^{f/f; TC} mice. n = 6 for each group. (C) Quantification of Tfr1 protein levels in mice that received a transplant of control or Stat5a/b^−/− fetal liver cells, expressed as the median measurements of α-CD71 fluorescence. n = 5 for each group. FL indicates fetal liver; PB, peripheral blood; and BM, bone marrow. *P < .05 compared with control mice. Error bars represent SEM. (D) Total RNA was extracted from Ter119-positive cells isolated from bone marrow and spleen. Tfr1 expression levels in control mice are set to 1. n = 3 in control group; n = 4 in Stat5a/b^{f/f; TC} group. *P < .05 compared with control mice. Error bars represent SEM.