Relative serum concentrations of Tpo from wild-type, NF-E2-null, and c-Mpl knockout (−/−) mice at various ages as determined in the bioassay using Tpo-dependent BaF/3 cells. Results are expressed as incorporated ³H-thymidine counts per well after 2 days of culture in medium supplemented with the mouse serum. rTPO, recombinant Tpo (saturating concentration, 10 ng/mL) used as an internal standard.