cS5 expression allows erythroid development in the absence of Epo. (A) E12.5 WT, EpoR^−/−, and Jak2^−/− fetal liver cells were infected with retroviruses encoding GFP or cS5 and cultivated in proliferation medium without Epo for 6 days. Cumulative cell numbers are shown for one representative experiments of 3. (B) Cytospins were prepared from cultures shown in panel A at day 4 ( in A) and stained with hematoxylin/eosin and benzidine. indicates apoptotic cells; ▶, pyknotic nuclei. (C) WT fetal liver cells expressing cS5 or GFP, or Jak2^−/− and EpoR^−/− cells expressing cS5, were subjected to CFU-E assays in the absence of Epo and acid benzidine–positive colonies scored at day 2. (D) Cytospins of cells retrieved from the CFU-E assays in (C), stained with hematoxylin/eosin and for hemoglobin (brownish).