EPO regulation of Egr1, Nab2, Nupr1, and Gspt1 via EPOR-H and EPOR-HM alleles. From wt-EPOR, EPOR-HM, and EPOR-H SP34-EX expanded marrow cell preparations, Kit^posCD71^high erythroblasts were isolated, washed, and plated in IMDM, 0.25% BSA, 25 ng/mL insulin, and 100 μg/mL transferrin. At 5.5 hours, cells were exposed to EPO (± 5 U/mL). At 90 minutes, RNA was isolated and was reverse transcribed. (A) Schematics of knocked-in minimal EPO receptor EPOR-HM and EPOR-H alleles. EPOR-HM is a PY-null allele, while EPOR-H selectively retains a PY343 Stat5 binding site. (B) For each erythroblast preparation, and EPOR allele, levels of EPO-modulated Erg1, Nab2, Nupr1, and Gspt1 transcripts were determined by quantitative PCR.