TfR^-/- ES cell clones and C57BL/6J chimera hemoglobin analysis. (A) Southern blot analysis of the parental TfR^+/- ES cell line and the 2 TfR^-/- ES cell clones obtained after neomycin step-up. (B) Isoelectric focusing of hemoglobins from peripheral blood of E12 TfR^-/- chimeras (CM-1 to CM-3), nonchimeric wild-type littermates, C57BL/6J (BL6), and 129 wild-type controls. M indicates marker lanes. (C) Hemoglobin electrophoresis of peripheral blood samples from TfR^+/- chimeras (het nos. 1-3), a 129 wild-type control, and 6 (of 8) H5 TfR^-/- chimeras. H5 chimeras no. 1 and no. 3 are not shown here, but in all cases, TfR^-/- chimeras show contribution only from the host C57BL/6J hemoglobin to circulating erythrocytes, indicating that the 129-derived TfR^-/- cells are unable to produce mature erythrocytes.