Expression of cS5 rescues proliferation and differentiation of EpoR^−/− and Jak2^−/− erythroid cells. (A) E12.5 WT, EpoR^−/−, and Jak2^−/− fetal liver cells infected with retroviruses encoding GFP (left panel) or cS5 (right panel) were cultivated in proliferation medium and cumulative cell numbers calculated after daily determination of growth rates. Data plotted for 1 typical experiment of 4. (B) Cytospins were prepared at day 6 from erythroid cultures shown in (A) and stained for hemoglobin (brownish color) plus histologic dyes. WT, Jak2^−/−, and EpoR^−/− fetal liver cells expressing cS5 or GFP were subjected to CFU-E- (C) or BFU-E assays (D), and acid benzidine–positive colonies were scored at day 2 (CFU-E) or day 8 (BFU-E), respectively. (E) Cytospins of cells retrieved from the CFU-E assays in (C), stained with hematoxylin/eosin and for hemoglobin (brownish color).