Fig. 4. Polycythemia is secondary to Epo-driven, bone marrow-derived red cell production. (A) Real-time RT-PCR reveals elevated Epo transcript levels in mutant liver and kidney at 3 weeks of age. The horizontal line indicates the median. *P<0.05; **P<0.01; ***P<0.001. (B) In vitro, FACS-based determination of late-progenitor and precursor erythroid cell differentiation to Ter119-positivity. Pcm/Pcm mice at 3 weeks of age show increased bone marrow erythroid activity at day 2. *P<0.001. (C) Increased hemoglobinization and cellularity of Pcm/+ bone marrow, as demonstrated by O-D staining of sections at 7 weeks of age. (D) In vitro analysis reveals increased bone marrow erythroid activity in Pcm/+ mice at 7 weeks of age. *P<0.001. (E) Real-time RT-PCR using 12-week kidney samples shows no statistically significant differences in Epo transcript levels among genotypes. The horizontal line indicates the median.