Insufficient transcription of erythroid regulator genes causes impaired erythropoiesis in Brd1^−/− fetal livers. (A) Quantitative RT-PCR analysis of expression of erythroid transcription factor genes in erythroblasts purified from wild-type and Brd1^−/− 12.5 dpc fetal livers. mRNA levels were normalized to Hprt1 expression. Expression levels relative to those in the wild-type erythroblasts are shown as the mean ± SE (n = 4∼5). (B) Rescue of defective proliferation of Brd1^−/− erythroblasts by exogenous Gata1. c-Kit⁺ CD71⁻ cells were sorted from wild-type (Brd1^+/+) and Brd1^−/− fetal livers at 12.5 dpc and cultured in the presence of SCF and IL-3. Twenty-four hours later, cells were infected with either GFP control or Gata1 retroviruses and the culture medium was changed to that containing EPO to induce erythroid differentiation. After a 3-day induction, cells were stained with the indicated antibodies and analyzed by flow cytometry. Cell growth during culture (left) and the final numbers of erythroid cells at different stages of differentiation (CD71⁻Ter119⁻ to CD71⁻Te119⁺; right) are shown as mean ± SE for triplicate cultures. ***P < .0005.