Id1 expression is dynamically regulated during terminal erythroid differentiation. (A) Ter119 and CD71 expression in E14.5 murine fetal liver cells as determined by FACS and showing 5 sequential stages of erythroid differentiation R1-R5. (B) Id1 transcript levels after 24 hours of in vitro culture of lineage-negative E14.5 fetal liver cells overexpressing Jak2 or Jak2V617F. (C) Four populations of murine E14.5 fetal liver cells: (i) Ter119⁻CD71⁻, (ii) Ter119⁻CD71⁺, (iii) Ter119⁺CD71⁺, and (iv) Ter119⁺CD71⁻, isolated by flow cytometry and representing sequential stages of erythroid differentiation. (D) Id1 transcript levels in the 4 sorted erythroid populations as determined by real-time quantitative PCR. Id1 transcript levels are bimodal with peaks of expression in ter119⁻CD71⁻ and Ter119⁺CD71⁺ cell populations.