Jak2^−/− cells expressing cS5 undergo erythroid differentiation in vivo. Equal numbers of cS5-transduced E12.5 WT and Jak2^−/− fetal liver cells were injected into sublethally irradiated mice. Six months after transplantation, spleen (A) and bone marrow (B) of engrafted animals were monitored for GFP-positive erythroid cells. FACS plots for Ter119 and GFP (left) indicate transplant-derived double-positive erythroid cells. Gated cells (boxed) were further analyzed for CD71 and Ter119 (right) to discriminate between immature (CD71^highTer119^pos) and mature (CD71^lowTer119^pos) erythroid cells.