Down-regulation of apoptosis in early splenic erythroblasts in ERK1^−/− mice. (A) Changes in Fas (n = 6 ERK1^+/+, n = 7 ERK1^−/−; left) and FasL (n = 10 ERK1^+/+, n = 11 ERK1^−/−; right) cell-surface expression on splenic erythroblasts from ERK1^−/− mice and ERK1^+/+ mice. The results are expressed as percentages of Fas/FasL-positive cells within each erythroblast subset. (B) Annexin V expression on splenic erythroblasts from ERK1-deficient mice (n = 6) compared with ERK1^+/+ mice (n = 6). The results are expressed as percentages of annexin V–positive cells within each erythroblast subset. Data are means ± SEMs (*P < .05; **P < .01)