Attenuated Stat5 activation and late-stage-specific decreases in Bcl-xL expression in Lyn^-/-erythroblasts. (A) Stat5 activation in Lyn^-/- erythroblasts is attenuated. Bone marrow-derived progenitor cells from Lyn^-/- and Lyn^+/+ mice were expanded in SP34-EX medium. Kit^negCD71^high and Kit^posCD71^high subpopulations of erythroblasts were isolated (by MACS), and washed cells were incubated in the absence of cytokines. Cells then were exposed to SCF plus Epo (150 ng/mL, 5 U/mL) and lysed at the indicated intervals. Levels of phospho- and total Stat5 then were assayed by Western blotting. In Lyn^-/- erythroblasts, levels of phospho-Stat5 were significantly decreased. In addition, levels of an approximately Mr 130 000 factor (^*), which crossreacted with the phospho-Stat5-specific antibody used in the present studies, also were diminished. (B) Bcl-xL expression is up-modulated in Kit^posCD71^high cells but decreased in Kit^negCD71^high and later-stage erythroblasts. In these erythroblast populations (i-ii), and in expanded Lyn^-/- and Lyn^+/+ erythroblasts that were differentiated for 1.5 days (iii), Bcl-xL levels also were analyzed (together with GAPDH).