STAT1^–/– erythroblasts show a block in erythroid differentiation. Cells were isolated from the (A) bone marrow or (B) spleen of wild-type and STAT1^–/– mice and incubated with PE-conjugated anti-Ter119 and biotin-conjugated anti-CD71 antibodies, followed by APC-conjugated streptavidin. Region I, proerythroblasts; region II, basophilic erythroblasts; region III, late basophilic and chromatophilic erythroblasts; and region IV, orthochromatophilic erythroblasts. The analysis was completed for 3 wild-type and 3 STAT1^–/– mice.